The third trimester of pregnancy begins around week 28. While the baby experiences its most rapid growth during this time, the overall rate of maternal weight gain often slows down. This deceleration is a common physiological event, particularly in the final weeks before delivery. This natural shift prepares the body for childbirth, contrasting with the steady accumulation of the second trimester.
The Typical Pace of Weight Gain in the Third Trimester
Weight gain during the first trimester is typically minimal, averaging only about one to four and a half pounds in total. The pace significantly increases during the second trimester, where a woman of healthy pre-pregnancy weight often gains approximately one pound per week.
In the third trimester, the expected weekly gain for a person with a normal pre-pregnancy Body Mass Index (BMI) typically ranges between 0.5 and 1 pound. As the due date approaches, often after week 36, the rate of gain can slow further or even plateau. It is not uncommon for some women to experience a slight weight loss of a pound or two just before labor as the body begins to shed excess fluid.
Breakdown of Third Trimester Weight Components
The baby experiences its most substantial growth spurt during this time, often adding five or more pounds to reach a birth weight of about seven to eight pounds. This rapid increase in fetal mass accounts for a significant portion of late-pregnancy weight.
The total weight gained is a combination of components. At term, the placenta weighs about one and a half pounds, and the amniotic fluid contributes approximately two pounds. The maternal body also retains a higher volume of blood and other bodily fluids, adding several pounds. The uterus itself grows considerably to accommodate the baby, contributing an additional two pounds of tissue.
Weight gain from maternal fat stores, accumulated earlier for energy reserves, increases more substantially during the second trimester than the third. This shift in composition—where maternal reserves slow while fetal weight accelerates—is what causes the overall rate of gain to decrease.
Physiological Factors That Influence Weight Gain
The slowing of weight gain is influenced by several physiological factors. The sheer size of the growing uterus places considerable pressure on the digestive organs, physically limiting food intake. This compression of the stomach often leads to early satiety, meaning a feeling of fullness after eating only a small amount of food.
Physical discomforts common in late pregnancy, such as heartburn or acid reflux, can further reduce appetite and caloric consumption. This discomfort results from the upward pressure exerted by the growing baby on the stomach. While the body’s metabolic rate increases to support the fetus, mechanical restriction on eating can sometimes counteract the need for increased calories.
As the body prepares for delivery, there is a natural regulation of fluid balance that can lead to a slight reduction in overall body weight. In the final weeks, amniotic fluid levels may decrease, and the body may excrete more accumulated water. Increased urination and perspiration contribute to this fluid loss, which can counterbalance the baby’s continued growth, resulting in the observed slowdown or plateau on the scale.
Monitoring Weight Gain and When to Consult a Doctor
Monitoring weight gain is an important aspect of prenatal care, guided by a woman’s pre-pregnancy BMI. Healthcare providers use these metrics to calculate a personalized target range for total weight gain throughout the pregnancy.
Rapid Weight Gain
A sudden, rapid increase in weight, especially after the 20th week, can be a sign of excessive fluid retention associated with conditions like preeclampsia. This rapid gain is often accompanied by other symptoms, such as severe headaches, visual changes, or significant swelling in the face and hands.
Cessation or Loss of Weight
Conversely, a complete cessation of weight gain or notable weight loss earlier in the third trimester may warrant investigation, as it could indicate an issue with fetal growth or maternal nutrition. While a gradual slowdown or slight loss in the final two to three weeks is generally considered normal, any abrupt or concerning changes should be discussed promptly with your healthcare provider.