influenced by smoking. Since the infants of smokers tend to have average birth weights that are approximately 200 g lower than those of the infants of nonsmokers (IOM, 1990) (which is the case in the study by Andersen et al. ), and since lower birth weight may decrease infant demand for milk and thus both prolactin levels and milk volume, it is difficult to separate cause and effect in these studies. Nonetheless, the evidence from investigations in both animals and humans strongly suggests that smoking has an adverse effect on milk volume.
The influence of alcohol consumption on milk production is less straightforward than that of smoking. It has long been maintained that small amounts of alcoholic beverages can help breastfeeding mothers to relax and thus foster effective functioning of the milk-ejection reflex (Lawrence, 1989). On the other hand, ethanol is a known inhibitor of oxytocin release (Fuchs and Wagner, 1963).
Two studies have demonstrated that the milk-ejection reflex can be at least partially blocked by maternal alcohol intake and that this effect is dose dependent (Cobo, 1973; Wagner and Fuchs, 1968). Wagner and Fuchs (1968) measured uterine contractions during suckling as an indicator of oxytocin release. At ethanol doses of 0.5 to 0.8 g/kg of maternal body weight, uterine activity was 62% of normal; at 0.9 to 1.1 g/kg, it was 32% of normal. Cobo (1973) measured the milk-ejection reflex by recording intraductal pressure in the mammary gland. He observed no effect of ethanol intake at doses below 0.5 g/kg; but the milk-ejection response was inhibited by 18.2, 63.2, and 80.4% at doses of 0.5 to 0.99, 1.0 to 1.49, and 1.5 to 1.99 g/kg, respectively. At 0.5 to 0.99 g/kg, this effect was not statistically significant, but at 1.0 to 1.49 g/kg, the milk-ejection reflex was completely blocked in 6 of the 14 subjects. The effect of alcohol on this reflex was not apparent when oxytocin was injected, indicating that the inhibition involved the release rather than the activity of oxytocin.
For an average woman weighing 60 kg (132 lb), an ethanol dose of 0.5 g/kg of body weight corresponds to approximately 2 to 2.5 oz of liquor, 8 oz of wine, or 2 cans of beer. Thus, these studies indicate that the adverse effects of alcohol consumption on the milk-ejection reflex are apparent only at relatively high intakes.
The impact of oral contraceptive agents on lactation performance has been the subject of numerous studies (see reviews by Koetsawang  and Lönnerdal ). In the United States, 12.6% of lactating women who participated in the 1982 National Survey of Family Growth reported that they