There are certain situations in which women will lose weight -- or maintain weight, but lose body fat -- while pregnant. Some women lose weight during their first trimesters, while others steadily lose body fat throughout their entire pregnancies. Depending upon the circumstances surrounding your weight loss, it may or may not have negative health effects.
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Pregnancy and Weight
In general, most women who are at a healthy pre-pregnancy weight need to gain about 25 to 35 lbs. in the course of their pregnancy. Drs. Michael Roizen and Mehmet Oz explain that this weight includes about 7 to 8 lbs. of extra body fat to serve as a maternal reserve that's used during breastfeeding. About 7 to 8 lbs. is due to the baby, and the remainder comes from the placenta and increased weight of the uterus, breasts and body fluid volume.
Typically, weight loss during pregnancy isn't healthy, but there are some times when it's considered much more normal -- and much less potentially harmful -- than others. For instance, explains Dr. Miriam Stoppard in her book "Conception, Pregnancy and Birth," weight loss isn't uncommon during the first trimester, due to the fact that many women experience some degree of morning sickness and reduce their food intake. First trimester weight loss is also much less detrimental to the baby's healthy than later weight loss.
Another potential cause of unintentional pregnancy weight loss is illness. If a woman is very ill -- either with continued morning sickness into the later trimesters, or with an infection -- during the second and third trimesters of her pregnancy, she may lose weight. Generally, later pregnancy weight loss is more serious than early weight loss, because the baby is larger and its nutritional needs are greater. Your obstetrician needs to know about it if you're losing weight during your second or third trimester.
For women who are quite overweight at the beginning of their pregnancies, most obstetricians don't advise actively trying to lose weight during pregnancy. However, many obstetricians will advise that these women gain no weight, meaning that to account for 17 to 27 lbs. of baby and baby-related organs and fluids, a woman will lose 17 to 27 lbs. of body fat, explain Drs. Roizen and Oz. As such, a very overweight woman can actually lose fat -- and therefore lose weight -- relative to her pre-pregnancy body.
Dr. Stoppard emphasizes that under no conditions should a pregnant woman try to lose weight on purpose during pregnancy, unless she has been advised to do so by her obstetrician. For the most part, physicians don't advise patients to diet during pregnancy for the goal of weight reduction, and attempting to lose weight can result in depriving your baby of the nutrition and energy it needs to grow and develop properly.