Muscles and joints shift and jostle. The heart’s pounding rhythm speeds up.  Blood roars through arteries and veins. Over the course of a pregnancy,  every organ in the body changes. Ignited by a range of hormones, these changes begin  as soon as pregnancy begins. Just days after fertilization, the embryo  implants in the lining of the uterus. Because its DNA doesn’t exactly match  the mother’s, the immune system should theoretically  recognize it as an invader, attack, and destroy it, like it would  bacteria or other harmful microbes. That’s the challenge:  the mother’s immune system needs to protect both her and the fetus,  but can’t act as it usually does. What happens is not as simple  as decreasing the immune response. Instead, it’s a complex interaction  we’re just beginning to understand, 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. As a pregnancy progresses, the uterus expands upward and outward  with the growing fetus. To make room, hormones  called progesterone and relaxin signal muscles to loosen. The muscles that propel food and waste  through the digestive tract also loosen, which makes them sluggish, 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. These changes can worsen morning sickness, which is caused in part by hormone HCG—  and can also happen at other times of day. As the uterus grows,  it pushes on the diaphragm, the muscle that expands and contracts  the chest with each breath. This limits the diaphragm’s range. To compensate, the hormone progesterone  acts as a respiratory stimulant, making the pregnant woman breathe  faster so both she and the baby can both get enough oxygen  with less lung capacity. This all may leave the pregnant woman  feeling short of breath. Meanwhile, 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. Usually, the body makes blood cells  using iron from our food. But during pregnancy, the fetus  is also building its own blood supply from nutrients in the mother’s food— leaving less iron and other nutrients  for the mother. 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— an important area of research, because some of the most serious  complications are related to the heart  and blood pressure. The expanding uterus may press on veins— causing fluid buildup  in the legs and feet. If it presses on a large vein  called the inferior vena cava, it might interfere with blood  returning to the heart, causing a dizzying drop in blood pressure  after standing for too long. Some of these changes start  to reverse even before birth. Shortly before delivery,  the fetus drops down, decreasing the pressure on the diaphragm and allowing the pregnant woman  to take deeper breaths. During labor and birth, much of the extra fluid in the body  is lost when the water breaks. The uterus shrinks back down  in the weeks after birth. Like the rest of the body,  pregnancy 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 lack of information about pregnancy’s  effects on the brain highlights a general truth: historically,  almost all the research around pregnancy has focused on the fetus,  rather than pregnant women. Experiences of pregnancy vary widely, both within the range  of healthy pregnancies and due to complicating health conditions— new research will help us understand why, and develop effective treatments  where necessary. In the meantime,  every pregnancy is different, and it’s important to consult a doctor  with any specific questions. Today, we’re turning an exciting corner, as more research is devoted  to the astounding biology of pregnancy.