Pregnancy begins when a sperm cell makes its way through the layers surrounding the egg and manages to fuse with it. This process involves two chemical reactions. First, the acrosomal reaction, where enzymes in the sperm’s head break down the egg’s outer layer. Then, once one sperm gets through, the cortical reaction kicks in. It instantly changes the egg’s surface to block any other sperm from entering to make sure only one set of paternal DNA gets in.

From there, the zygote starts dividing as it travels down the fallopian tube toward the uterus. These early divisions happen quickly; a blastocyst forms by the time it reaches the uterine wall. If all goes well, it implants into the thickened endometrium. That’s when the embryo begins releasing hCG, the hormone detected by pregnancy tests. hCG keeps the corpus luteum active so it continues producing progesterone, which maintains the uterine lining. Eventually, the placenta takes over hormone production to support the fetus and prevent the menstrual cycle from restarting.

The mother’s body changes during all of this. Blood volume increases, breathing rate changes, ligaments loosen to prepare for delivery, and metabolism changes to support the growing fetus. The uterus expands dramatically; by the third trimester, it can stretch from its normal size (about the size of a fist) to something that fills most of the abdominal cavity.

Pregnancy is usually divided into trimesters. In the first trimester (weeks 1–12), the embryo undergoes organogenesis, and the heart begins beating. This is also when many mothers feel the most fatigue, nausea, and hormonal shifts.

In the second trimester (weeks 13–26), the major organs continue developing, the skeleton begins to harden, and movements become noticeable. This is often when the pregnancy becomes physically visible, as the uterus expands and the belly begins to grow.

The third trimester (weeks 27–40) is all about growth and refinement. The fetus gains weight, the lungs mature, and space gets tight. The mother may feel increasingly tired or physically limited as the fetus presses on the bladder, diaphragm, and other organs.

Eventually, labor begins. The process is triggered by a complex hormonal cocktail: rising levels of estrogen, oxytocin, and prostaglandins. These chemicals prepare the cervix, stimulate uterine contractions, and help synchronize the body’s response.

Labor itself unfolds in three main stages. First, the cervix softens and dilates while contractions grow stronger and more regular. Then, in the second stage, the baby is pushed through the birth canal and delivered. The third stage is often overlooked, but just as important: the placenta detaches and is expelled shortly after birth.

Oxytocin has an important set of functions throughout. It strengthens contractions during labor and helps strengthen bonding after delivery. It’s also involved in lactation; once the baby starts suckling, oxytocin causes smooth muscle contractions that eject milk from the mammary glands. At the same time, prolactin is released, which promotes the ongoing production of milk. This creates a feedback loop where regular nursing helps sustain milk supply.