Pregnancy is the roughly 280-day journey from conception to birth that powers the continuation of the human species. After sperm meets egg, and the resulting blastocyst embeds in the uterine lining, the human body undergoes significant physiological changes to support the growth and development of this cluster of cells, enabling it to become an embryo and then a fetus.
Pregnancy triggers changes to every organ in a pregnant person's body. As the uterus expands, abdominal organs shift; the heart increases blood production; feet may grow larger; and brain areas heavily involved in processing information and emotions shrink by an average of 5%. Along the way, some women experience symptoms including nausea, mood swings, heightened sense of smell, congestion, and shortness of breath. In 2023, roughly 3.6 million babies were born in the United States.
In the US, miscarriage is the most common complication of pregnancy, occurring in as many as 20% of pregnancies—or roughly 1,000,000 cases annually. There are also around 21,000 stillbirths each year, and the same number of deaths during an infant's first year of life.
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Pregnancy is a complex biological journey, lasting about 40 weeks. When a sperm fertilizes an egg, that event triggers intricate cellular and hormonal processes, driven by hardwired cellular instructions, which lead to cell division, differentiation, and development—ultimately culminating in the birth of a baby.
The standard counting system considers a woman pregnant from the first day of her last menstrual period (LMP), which means someone may technically—for the purposes of measurement—have become pregnant even before she had sex. This standard starting point is used because it reliably marks when the body began preparing for pregnancy—even though it's typically two weeks before conception. On average, most women are pregnant for 280 days from the LMP.
This temporary organ, formed from maternal and embryonic tissue, ultimately attaches to a fetus via the umbilical cord. It transports nutrients, oxygen, antibodies, and more from the mother to the developing fetus. It also carries carbon dioxide and waste away from the fetus. The placenta—which only forms during pregnancy—is expelled during childbirth or it needs to be removed to prevent toxic maternal infections and extreme blood loss.
There's limited real estate inside a mother's body, so as the uterus grows both up and out it shifts other organs to make space. A pregnant woman's body produces hormones to loosen muscles to allow for this expansion. Pregnancy also triggers other changes, including faster breathing to compensate for decreased diaphragm range.
The brain areas involved in processing information and emotions decrease by an average of almost 5% during pregnancy. Researchers discovered in a 2025 study that this mostly temporary change—the brain hadn't fully returned to its normal size six months after pregnancy—seems to be associated with self-reported improved bonding and less hostility toward the child postpartum.
A hormone called relaxin loosens tissues in the body to allow them to expand during pregnancy, affecting muscles and body parts including the feet. The structural changes that occur in the foot as it expands are sometimes permanent. This article states that researchers have found as many as 61% of new moms did not have their foot sizes return to their pre-pregnant state.
This change, to support the developing fetus, puts stress on the heart and blood vessels. A pregnant woman's heart rate also increases as she pumps more blood each minute to support fetal growth.
A papyrus from about 1350 BCE discusses a test in which a woman would urinate on wheat and barley seeds over the course of several days and if the barley grows she's having a male child, if the wheat grows it's a female child, and if nothing grows she's not pregnant. Scientists went on to test this theory in 1963 and found that about 70% of the time, the urine of pregnant women did promote growth whereas the urine of non-pregnant women didn't—perhaps due to estrogen levels in the urine.
The study sought to test out the ancient Egyptian approach of having a woman urinate on barley and wheat seeds to ascertain pregnancy status. Surprisingly, the research concluded that a pregnant women's urine did promote growth of cereal grains the majority of the time and non-pregnant women's growth did not. Unfortunately, the sex of the child could not be accurately predicted based on which crop grew.
They pick up human chorionic gonadotropin, either in blood or urine. The hormone is produced by a woman's reproductive tissues during pregnancy.
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