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.