A standard for fetal growth in pregnancy [Classics Series]

1. After an initial rapid period of curvilinear growth, fetal weight was found to increase at a more linear rate through term.

Original Date of Publication: November 1976

This study summary is an excerpt from the book 2 Minute Medicine’s The Classics in Medicine: Summaries of the Landmark Trials, 2e (The Classics Series).

Study Rundown: As of the 1970s, there was not a standard for fetal growth in pregnancy. As such, it was not possible to adequately or reliably monitor growth, preventing optimal fetal monitoring and precluding insight into how numerous obstetric and medical conditions could affect the developing fetus. For example, the effects of maternal or obstetric pathology (e.g., chronic hypertension, preeclampsia, diabetes) on intrauterine growth could not be evaluated in the absence of a fetal growth standard. Additionally, diagnostic measurements of fetal growth, which can be used to estimate gestational age when a mother is not aware of her last menstrual period or the date of conception, were not possible. Previous investigations on the subject were limited by unreliable weight measurements of deceased neonates due to weighing of formalin-fixed fetuses. The largest prior study included 5000 Caucasian infants born to women of low socioeconomic status living at a high altitude such that results were not readily applicable to other populations. In the present work, authors compared fetal weight and growth measurements of tens of thousands of fetuses and newborns to determine a standard for intrauterine fetal growth.

This landmark study plotted the weights and gestational ages from more than 31 000 pregnancies to create the first standard for fetal growth in pregnancy appropriate for widespread use in the U.S. This was the first investigation to systematically evaluate growth standards by gestational age in a large and diverse population of American women. Strengths included the largest and most comprehensive assessment of fetal growth to date and use of information from 2 centers. Lack of accounting for biological sex, race, or maternal parity in fetal growth rate models may confound results. Though future investigations could improve upon this model, this classic initial investigation effectively demonstrated that after an initially rapid period of curvilinear growth, fetal weight increases in a more linear fashion through term.

Please click to read study in AJOG

In-Depth [retrospective cohort]: A total of over 31 268 fetuses and infants were assessed to characterize fetal growth rate, or weight gain per week of pregnancy. Data was obtained from 430 medical terminations performed at 8 to 21 weeks gestation in Chapel Hill and from 30 772 liveborn singleton infants delivered from 21 through 44 weeks estimated gestational age in Cleveland. All participants had reliable dating, defined as dating by last menstrual period consistent with clinician-measured fundal height. Percentiles of fetal weight gain by gestational age were plotted with 2-pointed weighted means. Simple linear regression was used to model crown-rump length by gestational age. Fetal growth rate is denoted by median fetal weight (grams, g) by estimated week of gestation.

The relationship between fetal growth and gestational age was initially curvilinear during a period of rapid growth in early pregnancy and then noted to become linear as fetal growth rate stabilizes. In contrast, weekly fetal weight gain increased throughout pregnancy to a maximum of just over 200 g/week from 34-36 weeks. At term, 37 weeks gestation, fetal weights were 2580 g at the 25th percentile, 2870 g at the 50th percentile, 3160 g at the 75th percentile, and 3470 g at the 90th percentile. The majority (> 50%) of infants at the premature gestational age of 30 weeks were low birthweight and weighed < 1500 g.

Brenner WE, Edelman DA, Hendricks CH. A standard of fetal growth for the United States of America. American journal of obstetrics and gynecology. 1976;126(5):555-64.

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