Much of the weight gain is due to the placenta, the fetus, the amniotic fluid and the energy reserves accumulated in anticipation of breastfeeding. The weight gain is related to all the body changes that occur to accommodate the baby.
Weight gain is a normal result of pregnancy. It promotes the development of the baby.
At the beginning of pregnancy and until the 15th week, the weight gain is primarily attributed to the mother. In fact, 1 to 2 kg is intended for protein reserves in the muscles and fat reserves that will be used later during the pregnancy. The fetus weighs as low as 65 g after 15 weeks, therefore the weight gain in early pregnancy is not directly associated to it.
Most of the weight gain occurs gradually during the 2nd and 3rd trimester, as the fetus and supportive tissue develop. It is for this reason also that the weight gain sought in the 1st trimester is only 1-2 kg (2.2 to 4.4 lbs).
The placenta, the fetus and amniotic fluid account for about 35% of total weight gain during pregnancy. This weight is lost during childbirth. The rest of the weight gain is due to increased blood volume, water retention, breast tissue and uterine tissue. Part of the energy reserves (fat reserves) accumulated during pregnancy is used during breastfeeding.
An adequate weight gain during pregnancy reduces complications during pregnancy and childbirth, improves the health of the mother in the long-term, promotes proper birth weight of the child and facilitates breastfeeding.
In sum, the weight gained by the mother includes the weight of the baby she carries, but also the structures that ensure the protection and development of the fetus (placenta, uterus, blood volume, amniotic fluid) in addition to preparing the mother for the postpartum period and breastfeeding.
In case of insufficient or significant weight gain, consult a health care professional like a physician and a nutritionist.