According to the U.S. Department of Health and Human Services, both pregnancy and labor can affect a woman's body. If your partner is trying to lose some additional pregnancy weight, make sure she does it in a healthy way and consults her doctor before she starts any type of diet or exercise plan.
If she wants to diet and is breastfeeding, it is best to wait until the baby is at least two months old. During those first two months, your spouse’s body needs to recover from childbirth and establish a good milk supply. Then when she starts to lose weight, she should try not to lose too much too quickly. This can be harmful to the baby because environmental toxins that are stored in the mother’s body fat can be released into her breast milk. Losing about one pound per week (no more than four pounds per month) has been found to be a safe amount and will not affect the milk supply or the baby's growth. The new mother can safely lose weight by consuming at least 1800 calories per day with a well-balanced, nutritious diet that includes foods rich in calcium, zinc, magnesium, vitamin B6, and folate. Diets in which she consumes less than 1500 calories per day are not recommended at any point during breastfeeding.
According to the U.S. Department of Health and Human Services, this can put her at risk for a nutritional deficiency, lower her energy level, and lower her resistance to illness.