Maternal smoking in pregnancy has been associated with both impaired fetal growth and perinatal deaths, suggesting abnormal placental function. The placenta, however, is disproportionately large in smokers at the time of delivery. It seemed possible that, because smoking reduces oxygen transport by increasing carboxyhemoglobin, placental enlargement might be a result of adaptive angiogenesis in the placental villi. The authors prepared plastic injection casts of the fetal capillaries within placental cotyledons, taking specimens from elective cesarean deliveries at 38 to 40 weeksrsquo; gestation. Four women who had smoked more than one pack a day throughout pregnancy and four nonsmokers were included in the study.Low-power microscopic study of the casts suggested increased capillary density within the terminal capillary convolutes in women who smoked (Fig. 1). Higher magnification disclosed increased branching within the terminal capillary loops in specimens from smokers. Both elongated and compact types of angiogenesis were observed. These findings point to an adaptive response of the fetal capillary bed within the placental villi of smokers, a change that should increase the surface area for diffusional exchange and thereby offset the impaired oxygen transport caused by carbon monoxide displacing oxygen from hemoglobin in maternal and fetal blood.Fig. 1. Representative scanning electronmicrograph images of terminal villous capillaries from nonsmoking (A, times;4700) and smoking (B, times;3760;C, 4700) placentas. Branches (arrowheads) and sinusoidal dilations (stars) are indicated. Note dense capillary network (C) and a high degree of coiling (B).VCequals; venous capillary. Reproduced with permission of The Lancet Ltd.Stopping smoking may be an important way of preventing perinatal death in women who are otherwise at low risk. It possibly would help to use a visual approach when trying to persuade women of the dangers of smoking while pregnant. They could be shown how the placenta tries to maintain oxygen transfer to the developing fetus.Lancet 1999;354colon;303
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