Since exposure to low winter temperatures during the second trimester of pregnancy, a period of rapid fetal weight gain, may result in lower birth weight, researchers suggest that pregnant women, particularly if they are at risk for premature delivery, take care to keep themselves warm during winter. In this analysis of 386,494 births occurring in Northern Ireland between 1971 and 1986, study authors found a clear seasonal variation in birth weights, although it was slightly more marked for females than for males. However, exposure to low temperature also raises plasma levels of fibrinogen, increases blood viscosity, and may induce vascular constriction in the placenta, all of which could diminish placental blood flow and reduce fetal growth.
Similar mechanisms appear to underlie low birth weight at high altitudes.