If you’re still breastfeeding and become pregnant, your mature breastmilk will transition back to colostrum around the end of the first trimester in preparation for whenever the new baby is born. By 4 months gestation, the placenta is large enough to suppress most milk production. Your body will prioritize your fetus over your nursling in terms of milk and produce the appropriate milk for the more vulnerable child. Colostrum is a high protein, laxative milk to help newborns poop out meconium. It is saltier in taste and thicker in consistency. Many older babies don’t care if it tastes different, but some will and may wean themselves. The supply is not likely to be able to support an infant 10 months old and younger. But if the older infant (11+ months) or toddler is eating solids and drinking other liquids, those babies may not care if they are getting milk or “dry nursing” until the next baby comes. Some choose to supplement younger babies with donor milk or may transition to formula until the new baby arrives and then continue to tandem feed with their own breastmilk again. 

Pregnancy hormones can make breasts and nipples more sensitive and uncomfortable. And these sensations often will make one want to wean or experience a nursing aversion. Toddlers may still aggressively want to nurse and it’s ok to put boundaries on your nursing. 

The typical things recommended to increase supply (additional feeding/pumping, herbs and supplements, etc.) are not appropriate and are ineffective since the placenta will continue to increase in size. Hormones supported by the placenta are what impact milk production and there’s not much you can do to combat the hormone shift as it’s needed to support the pregnancy.