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Fetuses Subsequently Born Premature Are Smaller Than Gestational Age–Matched Fetuses Not Born Premature

Departments of Radiology (P.M.D., C.B.B.), Obstetrics and Gynecology (L.W.-H.), and Newborn Medicine (S.R.), Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts USA.

Address correspondence and reprint requests to Peter M. Doubilet, MD, PhD, Department of Radiology, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115 USA.

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 = 1838) 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.

Key Words: birth weight • obstetric sonography • prematurity

Abbreviations: AD, abdominal diameter • ANCOVA, analysis of covariance • BPD, biparietal diameter • EFW, estimated fetal weight • FL, femur length • GA, gestational age