First-trimester growth and the risk of low birth weight

Background Previous studies have demonstrated a correlation between first-t rimester size and birth weight. it is not known, however, whether low birth weight is related to first-trimester growth. We sought to determine whethe r the risk of low birth weight and birth weight that was low for gestationa l age is related to the size of the embryo or the fetus in the first trimes ter. Methods From a data base of ultrasound records of more than 30,000 pregnanc ies, We identified women who had no important medical problems, a normal me nstrual history, and a first-trimester ultrasound scan in which the crown-r ump length of the embryo or fetus had been measured. We examined the relati on between the outcome of 4229 pregnancies and the difference between the m easured and the expected crown-rump length in the first trimester, expresse d as equivalent days of growth. Results A first-trimester crown-rump length that was two to six days smalle r than expected was associated with an increased risk las compared with a n ormal or slightly larger than expected crown-rump length) of a birth weight below 2500 g (relative risk, 1.8; 95 percent confidence interval, 1.3 to 2 .4), a birth weight below 2500 g at term (relative risk, 2.3; 95 percent co nfidence interval, 1.4 to 3.8), a birth weight below the fifth percentile f or gestational age (relative risk, 3.0; 95 percent confidence interval, 2.0 to 4.4), and delivery between 24 and 32 weeks of gestation (relative risk, 2.1; 95 percent confidence interval, 1.1 to 4.0), but not with delivery be tween 33 and 36 weeks (relative risk, 1.0; 95 percent confidence interval, 0.7 to 1.5). Conclusions Suboptimal first-trimester growth may be associated with low bi rth weight, low birthweight percentile, and premature delivery. (N Engl J M ed 1998;339:1817-22.) (C) 1998, Massachusetts Medical Society.