The Astounding Biology of Pregnancy: A Journey Through the Body

Pregnancy is a miraculous journey that transforms not only the life growing inside but also the body carrying that life. Every organ in the body changes, ignited by a range of hormones, and these changes begin as soon as pregnancy starts.

The Beginning of Pregnancy

Just days after fertilization, the embryo implants in the lining of the uterus. This is where the body’s immune system faces a challenge. The embryo’s DNA doesn’t exactly match the mother’s, so theoretically, the immune system should recognize it as an invader and destroy it. However, the body has a complex interaction involving many different types of immune cells, some of which seem to protect the fetus from attack by other immune cells. The body also creates an antibacterial plug made of mucus on the cervix, which keeps germs away and stays sealed until labor.

Changes in the Body

As pregnancy progresses, the uterus expands upward and outward with the growing fetus. To make room, hormones called progesterone and relaxin signal muscles to loosen. This loosening affects the muscles that propel food and waste through the digestive tract, causing constipation as passage through the tract slows down. Loosened muscles at the top of the stomach might allow acid to escape into the esophagus and throat, causing heartburn and reflux.

The uterus’s growth can also push on the diaphragm, the muscle that expands and contracts the chest with each breath. This limits the diaphragm’s range, causing the pregnant woman to breathe faster so both she and the baby can get enough oxygen with less lung capacity. This may leave the pregnant woman feeling short of breath.

Changes in Blood and Heart

During pregnancy, the kidneys make more erythropoietin, a hormone that increases red blood cell production. The kidneys also keep extra water and salt rather than filtering it out into urine to build up the volume of the blood. A pregnant woman’s blood volume increases by 50% or more. But it’s also a bit diluted, because it only has 25% more red blood cells. The heart has to work extra hard to pump all this blood through the body and placenta. A pregnant woman’s heart rate increases, but we don’t fully understand how blood pressure changes in a healthy pregnancy.

Effects on the Brain

Pregnancy also affects the brain, but its effects here are some of the least understood. Recent studies show differences in brain scans after pregnancy and early parenting, and suggest that these changes are adaptive. That means they could help with parenting skills, such as an increased ability to read facial cues since babies can’t talk.


The biology of pregnancy is astounding, with every organ in the body undergoing changes to accommodate and nurture the new life growing inside. As we continue to research and understand these changes, we can better support pregnant women and their unique needs during this transformative period.