Respiratory distress syndrome in twins

Single Nucleotide Polymorphisms Interactions of the Surfactant Protein Genes Associated With Respiratory Distress Syndrome Susceptibility in Preterm Infants

Background: Neonatal respiratory distress syndrome (RDS), due to surfactant deficiency in preterm infants, is the most common cause of respiratory morbidity. The surfactant proteins (SFTP) genetic variants have been well-studied in association with RDS; however, the impact of SNP-SNP (single nucleotide polymorphism) interactions on RDS has not been addressed. Therefore, this study utilizes a newer statistical model to determine the association of SFTP single SNP model and SNP-SNP interactions in a two and a three SNP interaction model with RDS susceptibility. Methods: This study used available genotype and clinical data in the Floros biobank at Penn State University. The patients consisted of 848 preterm infants, born <36 weeks of gestation, with 477 infants with RDS and 458 infants without RDS. Seventeen well-studied SFTPA1, SFTPA2, SFTPB, SFTPC, and SFTPD SNPs were investigated. Wang's statistical model was employed to test and identify significant associations in a case-control study. Results: Only the rs17886395 (C allele) of the SFTPA2 was associated with protection for RDS in a single-SNP model (Odd's Ratio 0.16, 95% CI 0.06-0.43, adjusted p = 0.03). The highest number of interactions (n = 27) in the three SNP interactions were among SFTPA1 and SFTPA2. The three SNP models showed intergenic and intragenic interactions among all SFTP SNPs except SFTPC. Conclusion: The single SNP model and SNP interactions using the two and three SNP interactions models identified SFTP-SNP associations with RDS. However, the large number of significant associations containing SFTPA1 and/or SFTPA2 SNPs point to the importance of SFTPA1 and SFTPA2 in RDS susceptibility.

Genetic Factors Contribute to Risk for Neonatal Respiratory Distress Syndrome among Moderately Preterm, Late Preterm, and Term Infants

OBJECTIVE

To determine the genetic contribution to risk for respiratory distress syndrome (RDS) among moderately preterm, late preterm, and term infants (estimated gestational age ≥32 weeks) of African- and European-descent.

STUDY DESIGN

We reviewed clinical records for 524 consecutive twin pairs ≥32 weeks gestation. We identified pairs in which at least 1 twin had RDS (n = 225) and compared the concordance of RDS between monozygotic and dizygotic twins. Using mixed-effects logistic regression, we identified covariates that increased disease risk. We performed additive genetic, common environmental, and residual effects modeling to estimate genetic variance and used the ratio of genetic variance to total variance to estimate genetic contribution to RDS disease risk.

RESULTS

Monozygotic twins were more concordant for RDS than dizygotic twins (P = .0040). Estimated gestational age, European-descent, male sex, delivery by cesarean, and 5-minute Apgar score each independently increased risk for RDS. After adjusting for these covariates, genetic effects accounted for 58% (P = .0002) of the RDS disease risk variance for all twin pairs.

CONCLUSIONS

In addition to environmental factors, genetic factors may contribute to RDS risk among moderately preterm, late preterm, and term infants. Discovery of risk alleles may be important for prediction and management of RDS risk.

What is the basis for a genetic approach in neonatal disorders?

Gene-environment interactions likely account for some degree of the variance in response rates that are clinically observed with antenatal corticosteroids, breast milk prophylaxis, surfactant administration, early recognition and treatment of sepsis, utility of non-invasive ventilation, and judicious exposure to supplemental oxygen. While these therapies and practice guidelines have significantly decreased overall neonatal mortality in the NICU, they have not made a marked impact on the frequency and severity of conditions such as bronchopulmonary dysplasia (BPD), necrotizing enterocolitis, and periventricular leukomalacia. One possible explanation is that genetic factors in the neonate modulate response to external intervention or preventative agents, culminating in variable levels of injury and different degrees of resolution and repair. Gene-environment explanations are supported by the observed heritability of BPD in twin studies, but they do not differentiate the interactions between neonate and offending toxin or pathogen, from interactions between neonate and intervention or therapeutic agent. Likely, both kinds of interactions are important in determining outcome.

Association of a FGFR-4 gene polymorphism with bronchopulmonary dysplasia and neonatal respiratory distress

Background. Bronchopulmonary dysplasia (BPD) is the most common chronic lung disease of premature birth, characterized by impaired alveolar development and inflammation. Pathomechanisms contributing to BPD are poorly understood. However, it is assumed that genetic factors predispose to BPD and other pulmonary diseases of preterm neonates, such as neonatal respiratory distress syndrome (RDS). For association studies, genes upregulated during alveolarization are major candidates for genetic analysis, for example, matrix metalloproteinases (MMPs) and fibroblast growth factors (FGFs) and their receptors (FGFR). Objective. Determining genetic risk variants in a Caucasian population of premature neonates with BPD and RDS. Methods. We genotyped 27 polymorphisms within 14 candidate genes via restriction fragment length polymorphism (RFLP): MMP-1, -2, -9, and -12, -16, FGF receptors 2 and 4, FGF-2, -3, -4, -7, and -18, Signal-Regulatory Protein α (SIRPA) and Thyroid Transcription Factor-1 (TTF-1). Results. Five single nucleotide polymorphisms (SNPs) in MMP-9, MMP-12, FGFR-4, FGF-3, and FGF-7 are associated (P < 0.05) with RDS, defined as surfactant application within the first 24 hours after birth. One of them, in FGFR-4 (rs1966265), is associated with both RDS (P = 0.003) and BPD (P = 0.023). Conclusion. rs1966265 in FGF receptor 4 is a possible genetic key variant in alveolar diseases of preterm newborns.

Genetic associations of surfactant protein D and angiotensin-converting enzyme with lung disease in preterm neonates

OBJECTIVE

To replicate genetic associations with respiratory distress syndrome (RDS) and bronchopulmonary dysplasia (BPD) in genes related to surfactant deficiency, inflammation and infection, and the renin-angiotensin system.

STUDY DESIGN

We examined eight candidate genes for associations with RDS and BPD in 433 preterm birth (PTB-<37 weeks) infants (251 with RDS and 134 with BPD). Both case-control and family-based analyses were performed in preterm (<37 weeks) and very preterm birth (VPTB-<32 weeks) infants.

RESULT

We replicated a previous finding that rs1923537, a marker downstream of surfactant protein D (SFTPD) is associated with RDS in VPTB infants in that the T allele was overtransmitted from parents to offspring with RDS (P=8.4 × 10(-3)). We also observed the A allele of rs4351 in the angiotensin-converting enzyme (ACE) gene was overtransmitted from parents to VPTB offspring with BPD (P=9.8 × 10(-3)).

CONCLUSION

These results give further insight into the genetic risk factors for complex neonatal respiratory diseases and provide more evidence of the importance of SFTPD and ACE in the etiology of RDS and BPD, respectively.

Surfactant protein-C promoter variants associated with neonatal respiratory distress syndrome reduce transcription

Dominant mutations in coding regions of the surfactant protein-C gene, SFTPC, cause respiratory distress syndrome (RDS) in infants. However, the contribution of variants in noncoding regions of SFTPC to pulmonary phenotypes is unknown. By using a case-control group of infants > or =34 weeks gestation (n = 538), we used complete resequencing of SFTPC and its promoter, genotyping, and logistic regression to identify 80 single nucleotide polymorphisms (SNPs). Three promoter SNPs were statistically associated with neonatal RDS among European descent infants. To assess the transcriptional effects of these three promoter SNPs, we selectively mutated the SFTPC promoter and performed transient transfection using MLE-15 cells and a firefly luciferase reporter vector. Each promoter SNP decreased SFTPC transcription. The combination of two variants in high linkage dysequilibrium also decreased SFTPC transcription. In silico evaluation of transcription factor binding demonstrated that the rare allele at g.-1167 disrupts a SOX (SRY-related high mobility group box) consensus motif and introduces a GATA-1 site, at g.-2385 removes a MZF-1 (myeloid zinc finger) binding site, and at g.-1647 removes a potential methylation site. This combined statistical, in vitro, and in silico approach suggests that reduced SFTPC transcription contributes to the genetic risk for neonatal RDS in developmentally susceptible infants.

The genetic susceptibility to respiratory distress syndrome

Previous studies to identify a genetic component to RDS have shown conflicting results. Our objectives were to evaluate and quantify the genetic contribution to RDS using data that comprehensively includes known environmental factors in a large sample of premature twins. Data from a retrospective chart review of twins born at < or =32 wk GA were obtained from two neonatal units. Mixed effects logistic regression (MELR) analysis was used to assess the influence of several independent covariates on RDS. A zygosity analysis, including the effects of additive genetic, common environmental and residual effects (ACE) factors, was performed to estimate the genetic contribution. Results reveal that the 332 twin pairs had a mean GA of 29.5 wk and birth weight (BW) of 1372 g. An MELR identified significant nongenetic covariates as male gender (p = 0.04), BW (p < 0.001), 5-min Apgar score (p < 0.001), and treating institution (p = 0.001) as significant predictors for RDS. The ACE model was used to estimate the genetic susceptibility to RDS by adjusting for the above factors. We found 49.7% (p = 0.04) of the variance in liability to RDS was the result of genetic factors alone. We conclude that there is a significant genetic susceptibility to RDS in preterm infants.

Surfactant protein polymorphisms and neonatal lung disease

Here, we describe the approach of defining the genetic contribution to disease and discuss the polymorphisms of some genes that are associated with respiratory disease. The common allelic variants of SP-A1, SP-A2, SP-B, SP-C, and SP-D genes are associated with respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), or respiratory syncytial virus (RSV) bronchiolitis. The main SP-A haplotype, interactively with SP-B Ile131Thr polymorphism and with constitutional and environmental factors, influences the risk of RDS. The polymorphisms of SP-A2 and SP-D are associated with the risk of severe RSV. The polymorphism may turn out to be important in susceptibility to influenza virus. The SP-B intron 4 deletion variant is the risk factor of BPD. Understanding the molecular mechanisms behind the hereditary risk may lead to new focused treatment strategies.

Genetic influences and neonatal lung disease

Neonatal lung diseases may have a genetic background. The available studies mainly concentrate on surfactant proteins (SP-A, SP-B) and respiratory distress syndrome. Specific alleles of the SP-A and SP-B genes associate interactively with susceptibility to respiratory distress syndrome. This genetic impact on the condition is influenced by environmental, acquired and inherited factors. Other alleles and genotypes of SP-A and SP-D associate with severe respiratory infections in early infancy. Rare mutations causing an absence of the SP-B protein result in progressive respiratory failure. Dominant mutations of SP-C associate with chronic lung disease, with variable manifestations. The first steps towards unraveling the genetic network influencing the susceptibility to neonatal lung diseases are now being taken. Genes encoding multifunctional proteins in the distal lung are prime candidates for causing susceptibility to neonatal lung disease, including bronchopulmonary dysplasia.

Surfactant proteins and genetic predisposition to respiratory distress syndrome

Respiratory distress syndrome (RDS) is caused by surfactant deficiency at birth. The risk of RDS decreases from the gestational age of 24 weeks to full-term. Genetic and acquired factors additionally influence the risk of RDS. Surfactant deficiency in RDS is mainly caused by immaturity and a lack of differentiation of the alveolar epithelial cells involved in surfactant synthesis and secretion. A network of hormones and growth factors regulate perinatal development. Host-related factors, including the levels of expression of surfactant proteins (SP), modulate the responsiveness of growth factors. SP-A has roles in surface activity and regulatory roles particularly in innate immunity; SP-B is essential for the processing of surfactant and for the surface activity; SP-C has roles in surfactant metabolism and function; the regulatory roles of SP-D mainly pertain to innate immunity. The genetic variation of SP-A and SP-B genes and the risk of RDS have been studied. Both SP-A and SP-B associate with susceptibility to RDS. The association between the SP-A allele and genotypes and the risk of RDS is dependent on the SP-B genotype and significantly influenced by the degree of prematurity, antenatal glucocorticoid therapy, multiple birth, and birth order. The alleles/genotypes of SP-A, SP-C, or SP-D also associate with several other inflammatory lung and airway diseases. Rare mutations in SP-B or SP-C cause serious, often fatal lung diseases. Genetic and post-genomic research is likely to eventually result in new diagnostic applications and specific therapies for the prevention of respiratory failure and inflammatory lung diseases.

Genetic influences in respiratory distress syndrome: a twin study

This is a twin study of the contribution of genetic influences on the pathogenesis of respiratory distress syndrome (RDS). Retrospectively, the files of twins born between 1976 and 1995 in the Academic Medical Centre (before Wilhelmina Gasthuis) of the University of Amsterdam were studied with a gestational age of 30 to 34 weeks and 1 or both with RDS. Data were collected on gestational age, birth weight, Apgar score, or diabetes in the mother as risk factors. All children were born vaginally. One hundred ninety-four pairs of twins were born with a gestational age of 30 to 34 weeks. Twenty-six pairs were excluded, because neither of the babies developed RDS. Of the 168 pairs, we were able to determine zygosity in 80 pairs, of which 18 were monozygotic and 62 were dizygotic twins. This is a normal ratio for the gestational age of 30 to 34 weeks. Risk factors as formulated above were not different between the monozygotic and dizygotic twins. RDS occurred more frequently in both twins when the twins were monozygotic (12 of 18, 67%) than when the twins were dizygotic (18 of 62, 29%). This difference was statistically significant, P < .05 by Chi square analysis. When only 1 twin developed RDS, it was the second born in 4 (67%) of the other 6 pairs of monozygotic twins and in 34 (77%) of the 44 dizygotic twins (77%). This twin study supports the notion of a genetic contribution to RDS.

Surfactant proteins as genetic determinants of multifactorial pulmonary diseases

Surfactant proteins, SP-A, SP-B, SP-C and SP-D, play important roles in pulmonary surfactant function and metabolism. SP-A and SP-D, being members of the collectin family of proteins, also interact with pathogens and are involved in pulmonary host defense. Respiratory diseases are among the most common causes of death worldwide. Several life-threatening lung diseases, such as neonatal respiratory distress syndrome (RDS) and acute ROS (ARDS), are associated with impaired surfactant function. Allelic variations of the SP-A and SP-B genes have been shown to be important genetic determinants in individual susceptibility to RDS, which is a good general model for a multifactorial pulmonary disease resulting from complex interactions between several environmental and genetic factors. Because SP-A and SP-D act directly in the clearance of common lung pathogens, the genes encoding these proteins have been implicated as candidates in a few infectious diseases, including respiratory syncytial virus (RSV) infections and tuberculosis.

Surfactant in respiratory distress syndrome and lung injury

A deficiency in alveolar surfactant due to immaturity of alveolar type II epithelial cells causes respiratory distress syndrome (RDS). In contrast to animals, the fetal maturation of surfactant in human lungs takes place before term, exceptionally large quantities of surfactant accumulating in the amniotic fluid. The antenatal development of surfactant secretion is very variable but corresponds closely to the risk of RDS. The variation in SP-A and SP-B genes, race, sex and perinatal complications influence susceptibility to RDS. Surfactant therapy has improved the prognosis of RDS remarkably. Abnormalities in alveolar or airway surfactant characterize many lung and airway diseases. In the acute respiratory distress syndrome, deficiencies in surfactant components (phospholipids, SP-B, SP-A) are evident, and may be caused by pro-inflammatory cytokines (IL-1, TNF) that decrease surfactant components. The resultant atelectasis localizes the disease, possibly allowing healing (regeneration, increase in surfactant). In the immature fetus, cytokines accelerate the differentiation of surfactant, preventing RDS. After birth, however, persistent inflammation is associated with low SP-A and chronic lung disease. A future challenge is to understand how to inhibit or redirect the inflammatory response from tissue destruction and poor growth towards normal lung development and regeneration.

Clinical biological and genetic heterogeneity of the inborn errors of pulmonary surfactant metabolism

Pulmonary surfactant is a multimolecular complex located at the air-water interface within the alveolus to which a range of physical (surface-active properties) and immune functions has been assigned. This complex consists of a surface-active lipid layer (consisting mainly of phospholipids), and of an aqueous subphase. From discrete surfactant sub-fractions one can isolate strongly hydrophobic surfactant proteins B (SP-B) and C (SP-C) as well as collectins SP-A and SP-D, which were shown to have specific structural, metabolic, or immune properties. Inborn or acquired abnormalities of the surfactant, qualitative or quantitative in nature, account for a number of human diseases. Beside hyaline membrane disease of the preterm neonate, a cluster of hereditary or acquired lung diseases has been characterized by periodic acid-Schiff-positive material filling the alveoli. From this heterogeneous nosologic group, at least two discrete entities presently emerge. The first is the SP-B deficiency, in which an essentially proteinaceous material is stored within the alveoli, and which represents an autosomal recessive Mendelian entity linked to the SFTPB gene (MIM 1786640). The disease usually generally entails neonatal respiratory distress with rapid fatal outcome, although partial or transient deficiencies have also been observed. The second is alveolar proteinosis, characterized by the storage of a mixed protein and lipid material, which constitutes a relatively heterogeneous clinical and biological syndrome, especially with regard to age at onset (from the neonate through to adulthood) as well as the severity of associated signs. Murine models, with a targeted mutation of the gene encoding granulocyte macrophage colony-stimulating factor (GM-CSF) (Csfgm) or the beta subunit of its receptor (II3rb1) support the hypothesis of an abnormality of surfactant turnover in which the alveolar macrophage is a key player. Apart from SP-B deficiency, in which a near-consensus diagnostic chart can be designed, the ascertainment of other abnormalities of surfactant metabolism is not straightforward. The disentanglement of this disease cluster is however essential to propose specific therapeutic procedures: repeated broncho-alveolar lavages, GM-CSF replacement, bone marrow grafting or lung transplantation.

Association between the surfactant protein A (SP-A) gene locus and respiratory-distress syndrome in the Finnish population

Respiratory-distress syndrome (RDS) in the newborn is a major cause of neonatal mortality and morbidity. Although prematurity is the most-important risk factor for RDS, the syndrome does not develop in many premature infants. The main cause of RDS is a deficiency of pulmonary surfactant, which consists of phospholipids and specific proteins. The genes underlying susceptibility to RDS are insufficiently known. The candidate-gene approach was used to study the association between the surfactant protein A (SP-A) gene locus and RDS in the genetically homogeneous Finnish population. In the present study, 88 infants with RDS and 88 control infants that were matched for degree of prematurity, prenatal glucocorticoid therapy, and sex were analyzed for SP-A genotypes. We show that certain SP-A1 alleles (6A2 and 6A3) and an SP-A1/SP-A2 haplotype (6A2/1A0) were associated with RDS. The 6A2 allele was overrepresented and the 6A3 allele was underrepresented in infants with RDS. These associations were particularly strong among small premature infants born at gestational age <32 wk. In infants protected from RDS (those that had no RDS, despite extreme prematurity and lack of glucocorticoid therapy), compared with infants that had RDS develop despite having received glucocorticoid therapy, the frequencies of 6A2 (.22 vs.71), 6A3 (.72 vs.17), 6A2/1A0 (.17 vs.68), 6A3/1A1 (.39 vs.10), and 6A3/1A2 (.28 vs.06) in the two groups, respectively, were strikingly different. According to the results of conditional logistic-regression analysis, diseases associated with premature birth did not explain the association between the odds of a particular homozygous SP-A1 genotype (6A2/6A2 and 6A3/6A3) and RDS. In the population evaluated in the present study, SP-B intron 4 variant frequencies were low and had no detectable association with RDS. We conclude that the SP-A gene locus is an important determinant for predisposition to RDS in premature infants.

Genetics of the hydrophilic surfactant proteins A and D

The use of candidate genes has increased the ability to identify genetic factors involved in diseases with complex and multifactorial etiology. The surfactant proteins (SP) A and D are involved in host defense and inflammatory processes of the lung, which are often components of pulmonary disease. Therefore, the SP-A and SP-D genes make particularly good candidates to study factors contributing to pulmonary disease etiopathogenesis. Moreover, SP-A also plays a role in the surface tension lowering abilities of pulmonary surfactant, which is essential for normal lung function. Although genetic variability at the SP-D locus may exist among humans, allelic variants have not yet been characterized. On the other hand, the human SP-A genes (SP-A1 and SP-A2) are characterized by genetically dependent splice variants at the 5' untranslated region and allelic variants. The polymorphisms that give rise to SP-A1 and SP-A2 alleles are contained within coding regions, potentially having an effect on protein function. There appears to be a correlation between SP-A genotype and SP-A mRNA content. Furthermore, one SP-A2 allele (1A0) shown to associate with low SP-A mRNA levels is found with higher frequency in a subgroup with respiratory distress syndrome. The evidence gathered thus far indicates that SP-A, possibly by interacting with other surfactant components, may play a role (e.g. be a susceptibility factor) in the development of respiratory disease.

Surfactant proteins: molecular genetics of neonatal pulmonary diseases

Genetic and phenotypic complexity has been described for diseases of varied etiology. Groups of patients with varied phenotype can be used in association studies as an initial approach to identify contributing loci. Although association studies have limitations, their value is enhanced by using candidate genes with functions related to disease. Surfactant proteins have been studied in the etiopathogenesis of neonatal pulmonary diseases. SP-A and SP-B polymorphisms are found at a higher frequency in certain groups of patients with respiratory distress syndrome (RDS), and SP-B mutations are linked to the pathogenesis of congenital alveolar proteinosis (CAP). Phenotypic heterogeneity is observed for both CAP and RDS. The available data suggest that a number of factors contribute to the etiology of CAP and RDS and, therefore, a multidisciplinary approach of clinical, genetic, epidemiologic, and statistical considerations is necessary for an in-depth understanding of the pathophysiology of these and other pulmonary diseases.

Association of pulmonary surfactant protein A (SP-A) gene and respiratory distress syndrome: interaction with SP-B

Deficiency of the lipoprotein complex, surfactant, can lead to respiratory distress syndrome (RDS) in the prematurely born infant. The surfactant proteins (SP) play important roles in the function of surfactant. Previously, we have characterized four allelic variants of the SP-A1 gene (6A, 6A2, 6A3, and 6A4) and five allelic variants of the SP-A2 gene (1A, 1A0, 1A1, 1A2, and 1A3). We hypothesized that specific SP-A alleles/genotypes are associated with increased risk of RDS. Because race, gestational age (GA), and sex are risk factors for RDS, we first studied the distribution and frequencies of SP-A alleles/genotypes while adjusting for these factors as confounders or effect modifiers in control (n = 86 white and 12 black subjects) and RDS (n = 106 white and 37 black subjects) populations with GAs ranging from 24 wk to term. Although the odds ratios of several alleles and genotypes were in the opposite directions for black and white subjects, the homogeneity of odds ratio reached statistical significance only in the case of 6A3/6A3. Although differences were observed in subgroups with different GAs (< or =28 and >28 wk) of the RDS white population, definitive conclusions cannot be made regarding the effect of modification by GA. No differences were observed as a function of sex. Second, we compared the frequencies of SP-A genotypes and alleles between control (n = 83) and RDS (n = 82) patients in the >28-wk white population. Differences between the two groups were observed for the 1A0 allele and 1A0 genotypes. Moreover, a significant synergistic positive association was observed between 1A0 allele + SP-B polymorphic variant and RDS. We conclude that 1) the genetic analyses of RDS and SP-A locus should be performed separately for black and white populations and 2) SP-A alleles/genotypes and SP-B variant may contribute to the etiology of RDS and/or may serve as markers for disease subgroups.

Dinucleotide repeats in the human surfactant protein-B gene and respiratory-distress syndrome

Pulmonary surfactant, a lipoprotein complex, is essential for normal lung function, and deficiency of surfactant can result in respiratory-distress syndrome (RDS) in the prematurely born infant. Some studies have pointed towards a genetic contribution to the aetiology of RDS. Because the surfactant protein B (SP-B) is important for optimal surfactant function and because it is involved in the pathogenesis of pulmonary disease, we investigated the genetic variability of the SP-B gene in individuals with and without RDS. We identified a 2.5 kb BamHI polymorphism and studied its location, nature and frequency. We localized this polymorphism in the first half of intron 4 and found that it is derived by gain or loss in the number of copies of a motif that consists of two elements, a 20 bp conserved sequence and a variable number of CA dinucleotides. Variability in the number of motifs resulting from either deletion (in 55.3% of the cases with the variation) or insertion (44.7%) of motifs was observed in genomic DNAs from unrelated individuals. Analysis of 219 genomic DNAs from infants with (n = 82) and without (n = 137) RDS showed that this insertion/deletion appears with significantly higher frequency in the RDS population (29.3 as against 16.8%, P < 0.05).

Factors associated with neonatal problems in twin gestations

We examined the neonatal outcome of 644 twins weighing 500 g or more and 656 singletons, born in the years 1984-1986 in the Soroka Medical Center, Beer-Sheva, Israel. There was nearly a four-fold risk of antepartum death in twins vs singletons, which disappeared when birth weight was controlled for. The risks for intrapartum and early neonatal mortality were not raised in this population. A statistically significant relative risk for congenital heart malformations in twins vs singletons remained (RR = 5.0, 95% CI = 1.5-16.3), after controlling for maternal age. Significantly higher rates of hyalin membrane disease, hypoglycemia, hyperbilirubinemia, anemia and septicemia were found in twins. Controlling for the confounding of the association between twinning and mortality or morbidity caused by differences in distributions of mode of delivery or gestational age between twins and singletons, was not as efficient as the controlling for birth weight. Thus, adjustment for birth weight removed all the excess risks detected except in hypoglycemia. Our findings suggest that the lower birth weight of twins, which is so intimately associated with multiple gestations, is probably the single most important factor associated with neonatal problems found in twin births.

Amniotic fluid phospholipid profile in multiple pregnancy and the effect of zygosity

Because respiratory distress syndrome (RDS) may occur in one twin but not the other it may be misleading to assess fetal lung maturity using amniotic fluid from only one sac. We compared the amniotic fluid lecithin/sphingomyelin (L/S), phosphatidyl glycerol/sphingomyelin (PG/S) and phosphatidyl inositol/sphingomyelin (PI/S) ratios between co-twins and co-triplets in 32 sets of twins and three set of triplets. In the twin pregnancies we found a weak correlation for L/S ratio but a much improved one for PG/S and PI/S. The concordance between sacs for all three ratios was better in monozygotic than in dizygotic twins. The efficacy of amniotic fluid PG in the determination of fetal lung maturity was demonstrated and the discrepancies between the sacs was much less for PG than for the L/S ratios. Employing the L/S ratio combined with the presence or absence of PG should reduce false results to a minimum.

Hyaline membrane disease in twins. A 7 year review with a study on zygosity

We reviewed 294 pairs of twins born from January, 1966 to December, 1972. In 19 pairs one or both members developed hyaline membrane disease (HMD). Of these, both twins were affected in 12 pairs, twin B alone in six pairs, and twin A alone in one pair. The group affected (19 pairs) had lower gestational age, birth weight, Apgar score, increased incidence of monozygotic (MZ) twins, and higher mortality rate than the group without HMD (275 pairs). MZ twins were more immature than dizygotic (DZ) twins (p less than 0.02). When both twins were affected they had lower gestational age, birth weight, and increased monozygosity than when B alone was affected (p less than 0.05). When twin B alone was affected, he had lower Apgar score than twin A (p less than 0.05). We suggested that (1) HMD occurs in twins because of lung immaturity, as it does in singletons; (2) monozygosity may be a predisposing factor to HMD because of the associated prematurity; and (3) the greater risk of twin B is probably related to birth asphyxia.