Obesity (defined as a body mass index, or BMI, greater than 30) in pregnant women is a high-risk condition associated with neural tube defects, delayed organ maturation, premature labor and stillbirths. It can also cause the fetus to grow larger than usual, requiring risky cesarean delivery.
According to Raul Artal, chairman of the department of obstetrics at St. Louis University School of Medicine, the stress hormones norepinephrine and epinephrine might mediate the effect of obesity on fetal development.
Pregnant women with hypertension have elevated levels of these hormones, which promote fat accumulation. Patrick Catalano, chairman of Obstetrics and Gynecology at the MetroHealth Medical Center in Cleveland, Ohio, explains that women who exercise during pregnancy and gain less weight appear to develop larger placentas to support the fetus.
One theory is that when pregnant women exercise, blood flows away from the uterus into other areas, such as muscles and skin, which might stimulate the placenta to grow to gain better blood access. The presence of a larger placenta could perhaps improve the fetus’s nutrient uptake. Babies born to women with this feature tend to be born with healthy levels of body fat.
Studies have even found a causal relationship between maternal obesity and childhood obesity, leading to a continuous cycle that’s important to break.
“Fitness is a way to prevent maternal complications and, secondarily, to prevent fetal complications,” explains Artal who recommends women get into a regular fitness program before becoming pregnant and then continue it throughout and after delivery.