Differing fetal growth patterns in pregnancies complicated by preterm labor

Spontaneous Preterm Birth: Is Prevention with Aspirin Possible?

Background The rate of preterm births in Germany is 8.6%, which is very high compared to other European countries. As preterm birth contributes significantly to perinatal morbidity and mortality rates, the existing prevention strategies need to be optimized and expanded further. About ⅔ of all women with preterm birth have preterm labor or premature rupture of membranes. They are bracketed together under the term “spontaneous preterm birth” as opposed to iatrogenic preterm birth, for example as a consequence of preeclampsia or fetal growth retardation. Recent studies suggest that low-dose aspirin does not just reduce the rate of iatrogenic preterm births but can also further reduce the rate of spontaneous preterm births. This review article presents the current state of knowledge.

Method A selective literature search up until April 2020 was done in PubMed, using the terms “randomized trial”, “randomized study”, “spontaneous preterm birth”, and “aspirin”.

Results Secondary analyses of prospective randomized studies on the prevention of preeclampsia with low-dose aspirin show that this intervention also significantly reduced the rate of spontaneous preterm births in both high-risk and low-risk patient populations. The results of the ASPIRIN trial, a prospective, randomized, double-blinded multicenter study carried out in six developing countries, also point in this direction, with the figures showing that the daily administration of 81 mg aspirin starting before 14 weeks of gestation lowered the preterm birth rate of nulliparous women without prior medical conditions by around 11% (11.6 vs. 13.1%; RR 0.89; 95% CI: 0.81 – 0.98, p = 0.012).

Conclusion Further studies on this issue are urgently needed. If these confirm the currently available results, then it would be worth discussing whether general aspirin prophylaxis for all pregnant women starting at the latest in 12 weeks of gestation is indicated.

Foetal growth velocities in twin pregnancies

A retrospective longitudinal study was performed to quantify foetal growth velocities in twin pregnancies and to determine the effect of variables specific to twin pregnancies on growth velocity. Foetal growth velocity standard deviation (Z) scores were calculated from serial ultrasound data using published singleton reference data for 131 consecutive sets of twins from 30 to 37 weeks' gestation. Compared with low-risk pregnancies, the twin foetal abdominal area growth velocity Z scores were significantly reduced from 30 to 37 weeks and biparietal diameter growth velocity Z scores were also significantly lower, from 30 to 33 weeks. Amongst the twin pairs there were no significant differences in Z scores with respect to chorionicity, foetal sex, birth order to whether delivery was premature or term. This retrospective study has demonstrated that twin foetal growth velocity is reduced when compared to singletons from at least as early as 30 weeks' gestation. Twin specific variables such as chorionicity, sex, birth order and subsequent premature birth do not need to be accounted for in the interpretation of growth velocities in twins. The clinical importance of determining foetal growth velocity in twin pregnancies awaits further prospective study.

Placental bed disorders in preterm labor, preterm PROM, spontaneous abortion and abruptio placentae

Failure of physiologic transformation of the spiral arteries has been studied using placental bed biopsies in several obstetrical syndromes. Contrary to what was originally believed, this lesion is not restricted to preeclampsia and/or intrauterine growth restriction. A review of published evidence indicates that failure of physiologic transformation can be observed in women with spontaneous second trimester abortions, preterm labor with intact membranes, preterm prelabor rupture of membranes and abruptio placentae. Therefore, disorders of deep placentation are present in a wide range of complications of pregnancy, emphasizing the importance of understanding the physiology and pathology of transformation of the spiral arteries. We propose that changes in the population and function of immunocytes at the maternal-fetal interface can be part of the mechanism of disease of obstetrical disorders, and this requires further investigation.

An episode of preterm labor is a risk factor for the birth of a small-for-gestational-age neonate

OBJECTIVE

Patients with an episode of preterm labor that subsides in response to tocolysis and who, subsequently, deliver at term are considered to have false preterm labor. However, the episode of "preterm labor" may represent the uterine response (ie, uterine contractions) to an insult not severe enough to trigger preterm parturition, but which may put the fetus at risk for additional pregnancy complications, including growth restriction. The objective of this study was to compare the frequency of small-for-gestational-age (SGA) neonates among patients with an episode of increased uterine contractility who delivered at term and those who delivered preterm.

STUDY DESIGN

This retrospective cohort study included 849 patients. Inclusion criteria were (1) regular uterine contractions that required hospitalization, (2) intact membranes, and (3) gestational age between 20 and 36 weeks. SGA was defined as a birth weight of <10th percentile for gestational age. Placental pathologic evidence was reviewed, and the results were used to classify patients into an inflammatory cluster, vascular cluster, or both. Contingency tables, Mann-Whitney U test, and multivariate logistic regression were used for statistical analyses. A probability value of <.05 was considered significant.

RESULTS

The prevalence of SGA neonates in the study population was 16.1% (124/772). Patients who delivered at term had a significantly higher frequency of SGA neonates than those who delivered preterm (21.5% [64/298] vs 12.7% [60/474]; P = .001); the results of placental pathologic evidence were available in 63.7% (492/772) of the patients. Patients who delivered at term had a higher frequency of fetal or maternal vascular lesions without histologic evidence of inflammation than those who delivered preterm (29.1 % [43/148] vs 18.9% [65/344]; P = .01). Term delivery after an episode of regular preterm uterine contractions was associated with an odds ratio of 2.22 (95% CI, 1.28-3.85) to deliver an SGA neonate after the statistics were controlled for confounding variables. A subanalysis limited to patients who received tocolysis showed similar results.

CONCLUSION

Patients with an episode of increased uterine contractility that subsided and who deliver at term are at risk for delivering an SGA neonate, which suggests that an episode of false preterm labor is not a benign condition. We propose that insults to the fetoplacental unit may be resolved by either irreversible preterm parturition or restriction of fetal growth.

The preterm parturition syndrome

The implicit paradigm that has governed the study and clinical management of preterm labour is that term and preterm parturition are the same processes, except for the gestational age at which they occur. Indeed, both share a common pathway composed of uterine contractility, cervical dilatation and activation of the membranes/decidua. This review explores the concept that while term labour results from physiological activation of the components of the common pathway, preterm labour arises from pathological signalling and activation of one or more components of the common pathway of parturition. The term "great obstetrical syndromes" has been coined to reframe the concept of obstetrical disease. Such syndromes are characterised by: (1) multiple aetiology; (2) long preclinical stage; (3) frequent fetal involvement; (4) clinical manifestations that are often adaptive in nature; and (5) gene-environment interactions that may predispose to the syndromes. This article reviews the evidence indicating that the pathological processes implicated in the preterm parturition syndrome include: (1) intrauterine infection/inflammation; (2) uterine ischaemia; (3) uterine overdistension; (4) abnormal allograft reaction; (5) allergy; (6) cervical insufficiency; and (7) hormonal disorders (progesterone related and corticotrophin-releasing factor related). The implications of this conceptual framework for the prevention, diagnosis, and treatment of preterm labour are discussed.

Fetal growth and onset of delivery: a nationwide population-based study of preterm infants

OBJECTIVE

This study was undertaken to assess whether deviations from normal fetal growth are associated with spontaneous preterm delivery.

STUDY DESIGN

A population-based study was performed, using Swedish Medical Birth Register data from 1991 through 2001. The total population comprised 1,007,648 singleton births. Intrauterine-derived growth standards were used to identify individual standard deviation (SD) from expected birth weight. Spontaneous preterm infants were compared with infants born after spontaneous labor at term. Results were obtained by using multiple logistic regression analysis.

RESULTS

Associations between smaller than population mean and spontaneous preterm birth were evident for all gestational age groups. The largest risk was found at 28 to 31 gestational weeks and birth weight less than -3 SD (OR: 13.3; 95% CI: 10.3-17.2). Spontaneous preterm infants born at 34 to 36 gestational weeks weighed 1 to 1.9 SD (OR: 1.1; 95% CI: 1.1-1.2) or 2 to 2.9 SD (OR: 1.6; 95% CI: 1.5-1.7) above the expected mean more often.

CONCLUSION

Deviation of fetal growth from the expected mean is associated with spontaneous preterm delivery.

Prematurity and fetal growth restriction

Assessment of the growth status of the fetus and neonate is an essential component of perinatal care. It requires a distinction to be made between physiological and pathological factors, and the prediction of the optimal growth that a baby can achieve in a normal, uncomplicated pregnancy. Such an individually customised standard can now be easily calculated by computer: it needs to be accurately dated, individually adjusted for physiological characteristics, exclude pathological factors such as smoking, and be based on a fetal weight trajectory derived from normal term pregnancies. Application of a customised standard to calculate the growth status of preterm babies gives us freshly insights into the causes of prematurity. Fetal growth restriction is seen as a strongly associated factor, which is often present before the onset of spontaneous preterm labour. This raises the question whether, in many instances, the initiation of parturition should be seen as a fetal adaptive response aimed at escaping an unfavourable intrauterine environment. These concepts have implications for the understanding of the pathophysiology of preterm labour, as well as its clinical management.

Impairment of growth in fetuses destined to deliver preterm

OBJECTIVE

The objective of this study was to test the hypothesis that fetuses destined to deliver preterm do not reach their individual growth potential.

STUDY DESIGN

In a case control design, data on 44 preterm deliveries at < or =34 weeks were compared with data on a control group of next consecutive term deliveries. Criteria for inclusion were dating by ultrasonography at <20 weeks and no medical or obstetric complications. For each fetus, GROW v.2 software was used to generate an individual optimal growth curve and to calculate the percentile of achieved growth potential for birth weight based on 6 independent factors (maternal weight, height, parity, ethnicity, fetal sex, and gestational age) identified as determining fetal weight from multivariate logistic regression analysis of 40,000 uncomplicated term pregnancies. Birth weight percentiles based on standard norms were also calculated for each fetus.

RESULTS

The number of fetuses with birth weight below the 5th, 10th, and 25th percentile of their growth potential was significantly higher in the preterm group (10, 13, and 18) compared with that in the control group (2, 2, and 6; P <.008, P <.001, and P <.008, respectively). There were no significant differences in variables defining growth potential between the case and control groups. The number of fetuses below the 5th and 10th percentile based on standard birth weight norms was not significantly different between preterm and term pregnancies (3 vs 1 and 5 vs 2; P =.37 and P =.27). Among preterm deliveries, those preceded by preterm premature rupture of the membranes had significantly fewer fetuses >75th percentile of their growth potential (2 vs 8; P =.025). Fetuses with lower gestational ages at preterm delivery achieved lower median percentiles of their growth potential.

CONCLUSION

A significant proportion of fetuses destined to deliver preterm do not reach their individual growth potential compared with those delivered at term. This finding challenges our concept of preterm delivery and management strategy aimed at tocolysis.

Birth weight references for twins

OBJECTIVE

We sought to formulate fetal and birth weight references for twins from longitudinal data.

STUDY DESIGN

This historic cohort study was based on 1831 pregnancies of twins born alive at >/=28 weeks' gestation from Baltimore, Maryland; Miami, Florida; Charleston, South Carolina; and Ann Arbor, Michigan.

RESULTS

When we compared singletons and twins, the percentiles of twins fell substantially below the 10th percentile of singletons by 28 weeks' gestation, below the 50th percentile by 30 weeks' gestation, and below the 90th percentile by 34 weeks' gestation. The difference at the 50th percentile was 147 g (10%) at 30 weeks' gestation, 242 g (14%) at 32 weeks' gestation, 347 g (17%) at 34 weeks' gestation, 450 g (19%) at 36 weeks' gestation, 579 g (22%) at 38 weeks' gestation, and 772 g (27%) at 40 weeks' gestation.

CONCLUSION

This new reference demonstrates that, although the overall pattern of fetal growth is slower for twins versus singletons from about 30 weeks' gestation, well-grown twins and singletons do not differ as much as previously believed.

Intra- and early postpartum ultrasonography: a review. Part I

The objective of this article is to review current literature pertaining to intra- and early postpartum sonography. All the manuscripts published in the English language regarding this topic were selected from a MEDLINE search from 1966 through August 1997. Additional sources were identified through cross-referencing. Currently, intra- and early postpartum sonography may be performed for either maternal or fetal indications. Maternal indications include cervical assessment in preterm labor/rupture of membranes, assessment of the lower uterine segment, size and position of uterine fibroids, guided-placement of central venous or pulmonary artery catheters, detection of intraoperative venous air embolism, deep venous thrombosis, assist management of the third-stage of labor, postpartum hemorrhage or febrile morbidity. Fetal indications include an anatomical survey in patients presenting without prenatal care, verification of fetal presentation, estimated fetal weight, assessment of the breech-presenting fetus, external cephalic version, management of delivery of the second-twin, and internal podalic version. Doppler flow velocimetry is a useful tool in depicting both maternal and fetal intrapartum physiologic changes. We conclude that intra- and early postpartum sonography is an established versatile diagnostic and interventional-guiding modality for many obstetric conditions and should be readily available on labor and delivery suites.

The relationship of ponderal index and other measurements to birthweight in preterm neonates

The objective was to determine the relationship between birthweight and anthropometric indices (PI and MAC/ OFC) in preterm infants. A group of 163 singleton preterm neonates born at 24-36 weeks were examined. Measurements were made of crown-heel length, midarm circumference, and occipito-frontal circumference. Birthweights were calculated as multiples of the mean (MoMs) for a given stage of gestation. There was a highly significant correlation between gestational age and MAC/OFC (p < 0.0001) but none with ponderal index. There was a significant correlation between weight expressed as multiples of the mean and both ponderal index (p < 0.008) and MAC/OFC (p < 0.0001). This relationship between birthweight and anthropometric indices suggests that measurement of the ponderal index does not provide a useful index of intrauterine nutrition.

Growth patterns in preterm and term twin deliveries

Preterm deliveries in singleton pregnancies have been shown to be associated with decreased rate of growth in utero. A total of 113 sets of twins were scanned serially at fortnightly intervals to establish fetal growth. There was no significant difference in the growth pattern in twins delivered before 37 weeks compared to those delivered after this time.