Sonographically estimated fetal weight percentile as a predictor of preterm delivery

The relationship between maternal body mass index and pregnancy outcomes in twin compared with singleton pregnancies

OBJECIVE

Women with twins have an a priori increased risk for many of the complications associated with maternal obesity. Thus, the impact of maternal obesity in twins may differ from that reported in singletons. In addition, given the increased metabolic demands in twin pregnancies, the impact of maternal underweight may be greater in twin compared with singleton gestations. Our objective was to test the hypothesis that the relationship between maternal pre-pregnancy body mass index (BMI) and adverse pregnancy outcomes differ between twin and singleton gestations.

METHODS

This was a retrospective population-based study of all women who had a singleton or twin hospital birth in Ontario, Canada, between April 2012 and March 2016. Data were obtained from the Better Outcomes Registry & Network (BORN) Ontario. The relationship between maternal BMI category and pregnancy complications was assessed separately in twin and singleton gestations. The primary outcome was a composite variable that included any of the following complications: preeclampsia, gestational diabetes, or preterm birth before 32^0/7 weeks. Relative risk (aRR) and 95% confidence intervals (CI) for adverse outcomes for each BMI category as defined by WHO (using normal weight category as reference) were generated using modified Poisson regression, adjusting for maternal age, nulliparity, smoking, previous preterm birth, and fetal sex.

RESULTS

A total of 487,870 women with singleton (n = 480,010) and twin (n = 7860) pregnancies met the inclusion criteria. The risk of the composite primary outcome, preeclampsia, gestational diabetes, and cesarean delivery increased with high maternal BMI in both singleton and twin gestations, but these associations were weaker in twin compared with singleton gestations (association of BMI ≥ 40.0 kg/m² with primary outcome: aRR = 3.10, 95%-CI 2.96-3.24 in singletons compared with aRR = 1.74, 95%-CI 1.37-2.20 in twins). In singleton pregnancies the risk of preterm birth at < 32^0/7 weeks increased with maternal BMI, mainly due to an increased risk of provider-initiated preterm birth. In twin gestations, however, underweight (but not overweight or obesity) was associated with the greatest risk of preterm birth at < 32 weeks (aRR 1.67, 95%-CI 1.17-2.37), mainly due to an increased risk of spontaneous preterm birth (aRR 2.10, 95%-CI 1.44-3.08).

CONCLUSION

In healthy women with twin pregnancies, underweight is associated with the greatest risk for preterm birth, while the association of maternal obesity with adverse pregnancy outcomes is weaker than that observed in singletons.

A modified prenatal growth assessment score for the evaluation of fetal growth in the third trimester using single and composite biometric parameters

OBJECTIVE

To define modified Prenatal Growth Assessment Scores (mPGAS) for single and composite biometric parameters and determine their reference ranges in normal fetuses.

METHODS

Nine anatomical parameters (ap) were measured and the weight estimated (EWTa, EWTb) in a longitudinal study of 119 fetuses with normal neonatal growth outcomes. Expected third trimester size trajectories, obtained from second trimester Rossavik size models, were used in calculating Percent Deviations (% Dev's) and their age-specific reference ranges in each fetus. The components of individual % Dev's values outside their reference ranges, designated +iapPGAS, -iapPGAS, were averaged to give +apPGAS and -apPGAS values for the 3rd trimester. The +iapPGAS and -iapPGAS values for different combinations of ap (c1a (HC, AC, FDL, ThC, EWTa), c1b (HC, AC, FDL, ThC, EWTb), c2 (ThC, ArmC, AVol, TVol), c3 (HC, AC, FDL, EWTa)) were then averaged to give +icPGAS and -icPGAS values at different time points or at the end of the third trimester (+cPGAS, -cPGAS). Values for iapPGAS, ic1bPGAS, and ic2PGAS were compared to their respective apPGAS or cPGAS reference ranges.

RESULTS

All mPGAS values had one 95% range boundary at 0.0%. Upper boundaries of 1D +apPGAS values ranged from 0.0% (HC) to +0.49% (ThC) and were +0.06%, +2.3% and +1.8% for EWT, AVol and TVol, respectively. Comparable values for -apPGAS were 0.0% (BPD, FDL, HDL), to -0.58% (ArmC), -0.13% (EWT), -0.8% (AVol), and 0.0% (TVol). The +cPGAS, 95% reference range upper boundaries varied from +0.36% (c1b) to +0.89% (c2). Comparable values for -cPGAS lower boundaries were -0.17% (c1b) to -0.43% (c2).

CONCLUSIONS

The original PGAS concept has now been extended to individual biometric parameters and their combinations. With the standards provided, mPGAS values can now be tested to see if detection of different types of third trimester growth problems is improved.

Diagnosis of IUGR: traditional biometry

An important advance in obstetric medicine will be the improved ability to identify pathologic states of fetal growth, determine their consequences, and implement appropriate interventions. In response to utero-placental insufficiency and under nutrition, the fetus makes physiologic, metabolic, and hormonal adaptations which influence growth, including reducing metabolic dependence on glucose and increasing oxygenation of other nutritional substrates including amino acids and lactate. These endocrine changes combined with reduced nutrient supply divert amino acids from protein synthesis and tissue growth, resulting in impaired somatic growth and diminished growth of kidneys, liver, and heart-the developing organs with the highest rates of cellular turnover. The obstetrician must be able to recognize and accurately diagnosis the fetus with intrauterine growth restriction (IUGR). Ultrasonography is the accepted standard for monitoring fetal growth. Serial ultrasound measurements can provide a reasonable estimate of fetal gestational age and weight based on individual and composite fetal biometric measurements. The purpose of this chapter is to discuss those traditional biometric measurements as they relate to the diagnosis of IUGR.

Fetal growth rates and the very preterm delivery of twins

OBJECTIVE

Our objective was to determine whether slow fetal growth rates and twin growth patterns from 20 weeks' gestation to delivery are associated with very preterm delivery.

STUDY DESIGN

Available charts were reviewed for twin pregnancies, delivered between 1979 and 2002, at 4 U.S. medical centers. The sample of 1612 pregnancies delivered at 28 week's gestation or greater and had at least 2 ultrasound evaluations of fetal size from 20 to 28 weeks or from 28 weeks to delivery for estimation of fetal growth rates (grams per week). Slow fetal growth (below the 10th percentile) was defined as less than 90 grams per week at 20-28 weeks and 168 g/week from 28 weeks to delivery. The main outcome measure was the timing of delivery.

RESULTS

Of the women delivering twins, 5.3% delivered extremely preterm (28-30 weeks), 8.5% very preterm (31-32 weeks), and 40.1% preterm (33-36 weeks). Patterns of growth for the pair were highly associated with very preterm delivery. Compared with neither growing slowly (1.7%), 4.9% delivered very preterm if only 1 twin grew slowly. Very preterm was 14.6% (adjusted odds ratio 9.81; 95% confidence interval, 3.50-27.48) with both growing slowly from 28 weeks on and 20.0% (adjusted odds ratio 15.04; 95% confidence interval 5.13-44.11) with both growing slowly over both intervals. Survival analyses indicated that twins with normal growth in both intervals remained undelivered for a significantly longer number of days (P < .0001) than pairs in which one or both twins were growing slowly.

CONCLUSION

Very preterm delivery of twins seems to be preceded by slowed or compromised fetal growth for the pair.

Risk factors for adverse outcomes in spontaneous versus assisted conception twin pregnancies

OBJECTIVE

To evaluate risk factors for adverse outcomes in spontaneous vs. assisted conception twin pregnancies.

DESIGN

Historical cohort study.

SETTING

Four academic tertiary medical centers.

PATIENT(S)

Women with twin pregnancies, including 2,143 spontaneous and 424 assisted conception; 2,492 nonreduced and 75 reduced.

INTERVENTION(S)

None (observational).

MAIN OUTCOME MEASURE(S)

Preeclampsia, preterm premature rupture of membranes, birth <32 weeks and <30 weeks, low birth weight, very low birth weight, and slowed midgestation fetal growth (<10th percentile between 20 and 28 weeks).

RESULT(S)

Among nonreduced pregnancies, assisted conception was not significantly associated with any adverse outcomes; among nulliparas, the risk for preeclampsia was increased regardless of method of conception; among spontaneous conceptions, the risks for preterm premature rupture of membranes, low birth weight, very low birth weight, and slowed midgestation fetal growth were increased. Among all pregnancies, fetal reduction increased risks for birth <32 weeks and <30 weeks, low birth weight, very low birth weight, and slowed midgestation fetal growth. Among nulliparas with assisted conceptions, fetal reduction increased the risks for birth <30 weeks, very low birth weight, and slowed midgestation fetal growth.

CONCLUSION(S)

These findings indicate that in twin pregnancies, assisted conception is not a risk factor for adverse outcomes, but rather specific factors that are more common among these pregnancies, such as nulliparity and fetal reduction, increase risks.

Fetuses subsequently born premature are smaller than gestational age-matched fetuses not born premature

OBJECTIVE

To determine whether singleton fetuses in pregnancies that subsequently deliver prematurely are smaller than singleton fetuses of the same gestational age who are not born premature.

METHODS

Our study population consisted of singleton pregnancies meeting the following criteria: at least 1 first-trimester sonogram (to ensure accurate dating), at least 1 sonogram after 24 weeks' gestation, and a known delivery date. Controlling for gestational age at sonography using analysis of covariance, we compared very premature fetuses (delivery at 24-29.9 weeks; n = 26) and moderately premature fetuses (delivery at 30-36.9 weeks; n = 306) with nonpremature fetuses (n = 1,838) with respect to the following sonographic parameters: abdominal diameter, femur length, biparietal diameter, and estimated fetal weight.

RESULTS

On 24- to 29.9-week sonograms, fetuses who were subsequently born very premature had significantly smaller fetal measurements than did fetuses who were not born premature (P < .05 for all parameters). Even after excluding 9 very premature fetuses with a risk factor for uteroplacental insufficiency (e.g., toxemia or hypertension), the remaining 17 fetuses had significantly smaller abdominal diameters, femur lengths, and estimated fetal weights than nonpremature fetuses (P < .05). Sonographic parameters in moderately premature fetuses were smaller than in nonpremature fetuses on 30- to 36.9-week sonograms (P < .05 for all parameters), but the only parameters that differed significantly between these 2 groups on 24- to 29.9-week sonograms were abdominal diameter and femur length. CONCLUSIONS. Singleton fetuses subsequently born premature are smaller than gestational age-matched fetuses not born premature, even in the absence of an identifiable cause of growth restriction. The lag in growth appears to occur in the last few weeks before delivery.