According to The Breastfeeding Answer Book, the hormones that maintain pregnancy are found in only very small quantities in mother's milk, and they are not harmful to the breastfeeding child. Many nursing babies will easily and happily adjust to the change, while others may start to lose interest and begin the weaning process. Your best bet is to arm yourself with information on how to recognize and support child-led weaning and heed your little one's cues. While there have been studies showing that nipple stimulation can indeed initiate labor in the third trimester, the types of mechanical stimulation in the studies were more vigorous than one would get from a nursing child. Again, mothers with a history of preterm labor or miscarriage are often advised to wean when pregnant.
Some scientists theorize that mothers experience a biological impulse to wean during pregnancy.
Fifth International Congress on Psychosomatic Obstetrics and Gynecology. Similarly, pregnant women often experience tender breasts and sore nipples that become incredibly painful while nursing. Many such women report, however, that the pain is most intense during the first 15 seconds or so of a nursing session and then drops off. Try varying positions and employing relaxation and breathing techniques. And rest assured that the pain will lessen after the first trimester when your nipples typically become less sensitive.