Low Birth-Weight Risk Begins Early

December 16, 1998

Low birth-weight risk begins early in first trimester, Cornell pregnancy researcher reports

ITHACA, N.Y. -- Reviewing thousands of ultrasound scans at a Scottish hospital, pregnancy researchers found that many cases of extremely premature delivery and poor fetal growth are likely to be determined as early as the first 12 weeks of pregnancy.

The finding may indicate that waiting until after conception to prepare for motherhood may be waiting too long.

Results of the ultrasound survey, which measured the fetal size of babies born at the Queen Mother's Hospital in Glasgow over a 10-year period, are being reported tomorrow (Dec. 17, 1998) in the "New England Journal of Medicine" by Gordon C.S. Smith, M.D., of the Laboratory for Pregnancy and Newborn Research in Cornell University's College of Veterinary Medicine, together with Malcolm F.S. Smith, Margaret B. McNay and John E.E. Fleming, all of the Department of Obstetrics and Gynecology at the University of Glasgow. The study was supported, in part, by the Wellcome Trust.

"We used to think that variations in birth weight were determined in the second half of pregnancy. Now it appears that conditions quite early in gestation -- or perhaps even before conception -- are contributing to low birth weight and the risk of extremely premature birth," says Cornell's Smith.

"We don't yet have enough information to recommend first-trimester ultrasound measurement as a diagnostic tool for extremely premature birth or fetal-growth restriction," he cautions. "Nevertheless, this finding could help explain why programs of nutritional supplementation for expectant mothers deemed to be at high risk of a low birth-weight baby typically only increase the baby's weight by 2 or 3 ounces." Poor fetal growth might be determined before the woman knows she is pregnant or begins to receive prenatal care, he says.

The researchers began with the medical records for more than 31,000 pregnancies at the Glasgow hospital and eliminated from consideration all with complicating factors, such as illness in the mother or fetal abnormalities. They also excluded women who were uncertain about their conception dates or who had not received their first prenatal care until the second or third trimester. The analysis was then focused on the remaining 4,229 pregnancies.

Electronic calipers were used with each ultrasound image to measure the crown-rump length (the entire head and body, minus the legs), and each pregnancy was followed through to delivery. In a statistically significant number of cases, the researchers report, embryos (the first eight weeks of pregnancy) and fetuses (beyond eight weeks) that were small for their age during the first trimester were twice as likely to be born prematurely and three times more likely to fall into the smallest 5 percent of babies.

"A significant proportion of low birth-weight and extremely premature birth seems to be determined at a very early stage," Smith comments. "Obstetricians can't generally predict extremely premature delivery and when a baby is born prematurely there is often no obvious cause. Our study suggests that inadequate transfer of nutrients to the growing fetus may explain some of these cases."

The finding that low birth weight and premature birth is determined in the first trimester, Cornell's Smith says, might indicate a benefit that could result from even earlier intervention. He suggests better nutrition and health care for women of child-bearing age who are likely to become pregnant. Waiting until after conception to prepare for motherhood might be waiting too long, he says.

Attempts by mothers to eat heartily in the first trimester might be futile, Cornell's Smith says, pointing to a maternal paradox: "Mothers don't wish to eat much in the early weeks. One theory is that a mother's nausea protects her developing embryo from natural toxins in food," he says. "And this may be a way of reminding mothers to protect the unborn from unnatural toxins, like cigarette smoke, too."

Cornell University

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