Smoking, hemoglobin levels, and birth weights in normal pregnancies.

Of 811 otherwise healthy, nonanemic, pregnant Norwegian women, 43% smoked during the third trimester. No significant association existed between smoking habits and hemoglobin levels during the third trimester, but the fairly small number of heavy smokers (greater than 10 cigarettes per day) may have precluded the discovery of subtle true differences. Birth weights were transferred to a weight-for-dates percentile scale, based on the distribution of the total Norwegian birth population over an 11-year period. With a grouping in quartiles according to this scale, a strong correlation between smoking habits and low birth weights emerged, with 47% of newborn infants of heavy smokers falling in the lowest quartile. Levels of hemoglobin were inversely correlated to birth weight quartiles, equally in mothers who smoked and those who did not smoke. However, when birth weights were grouped according to maternal hemoglobin at term, the tobacco-related effect on birth weight appeared only with levels of hemoglobin above 12 gm/dl, being particularly strong for levels above 13 gm/dl. Hemoglobin levels of 9.0 to 11.9 gm/dl appeared to protect from growth retardation the fetuses of nonanemic mothers who smoked. Studying ponderal, or weight-by-length, indices, we found that growth-retarded babies of mothers who smoked tended to be thin, thus indicating that they had been deprived of nutrients.