Trouble With the Curve: Pearls and Pitfalls in the Evaluation of Fetal Growth

Poor fetal growth is one of the most important findings on an obstetric ultrasound (US) examination. First-trimester US is the most accurate means for dating pregnancies; however, dating based on last menstrual period remains the standard for determining gestational age. Discrepancies between menstrual and US dating can lead to the overdiagnosis of in utero growth restriction (IUGR). This article will demonstrate that as long as the fetal growth falls along a curve that parallels normal growth curves, appropriate growth has occurred regardless of the gestational age and weight percentile that has been assigned to the fetus.

Following Institutional Review Board approval (HSD-00002969), 860 third-trimester fetal US examinations were retrospectively evaluated from January 1 through July 1, 2017, to determine whether they had normal growth curves. Outcome data on all cases were obtained from review of the medical records.

Of 216 fetuses (25%) suspected of IUGR based on a weight below the 10th percentile, 6 developed true IUGR: 5 that led to emergent delivery of fetuses weighing less than a 1000 g and 1 in utero fetal demise. The remaining 210 fetuses all had normal outcomes.

As long as the fetal growth falls along a curve that parallels normal growth curves, appropriate growth has occurred regardless of the gestational age and weight percentile that has been assigned to the fetus.