Pregnancy involves a surge of hormones that build and sustain a new life from conception through delivery. Some are unique to pregnancy, produced only by the placenta. Others are familiar hormones like estrogen and progesterone, just at dramatically higher levels than your body has ever experienced. Together, they control everything from implantation to labor contractions to milk production. Here’s what each one does and how it affects you.
hCG: The Hormone That Signals Pregnancy
Human chorionic gonadotropin, or hCG, is the hormone detected by pregnancy tests. It’s produced by cells that eventually form the placenta, and it appears in your blood and urine within days of a fertilized egg implanting in the uterine wall. In early pregnancy, hCG levels almost double every three days. They peak around weeks 9 to 10, then gradually decline for the rest of the pregnancy.
The primary job of hCG is to keep the corpus luteum alive. The corpus luteum is a small structure on the ovary that forms after ovulation, and it produces progesterone, the hormone your body needs to maintain the uterine lining. Without hCG signaling the corpus luteum to keep working, progesterone would drop and the pregnancy couldn’t continue. Once the placenta takes over progesterone production (around 10 weeks), hCG becomes less critical.
hCG is also the hormone most strongly linked to morning sickness. The timing lines up: nausea and vomiting tend to peak between weeks 12 and 14, right alongside hCG levels. Studies have found that higher hCG concentrations correlate with more severe nausea, though not every study agrees on the strength of that link. Estrogen and progesterone may contribute too, but the evidence is less consistent.
Progesterone: Keeping the Pregnancy in Place
Progesterone is often called the “pro-gestation” hormone for good reason. Its most important job is maintaining a calm uterus. It reduces the tone and contractility of the uterine muscle, which prevents the body from contracting and expelling the developing embryo. This suppression of uterine contractions continues throughout pregnancy, keeping the fetus safely in place until labor begins.
For the first 10 weeks, progesterone comes from the corpus luteum on the ovary. After that, the placenta becomes the primary source and keeps levels elevated until delivery. Beyond the uterus, progesterone also plays a role in suppressing lactation during pregnancy. Your breasts are preparing to produce milk, but progesterone helps block that process until after birth, when its levels plummet.
Progesterone is behind many of the less glamorous symptoms of pregnancy: bloating, constipation, and fatigue. Because it relaxes smooth muscle throughout the body (not just in the uterus), it also slows digestion and can contribute to heartburn.
Estrogen: Fueling Growth and Blood Flow
Estrogen levels rise steadily throughout pregnancy, reaching concentrations far higher than at any other time in your life. One of its key roles is increasing uterine blood flow, which ensures the placenta receives enough oxygen and nutrients to support the growing fetus. It also works alongside progesterone and prolactin to stimulate breast tissue development, building the structures that will eventually produce milk.
Estrogen plays a protective role for the fetus as well. It helps regulate a placental enzyme that converts the stress hormone cortisol into an inactive form, shielding the baby from excessive exposure. Too much cortisol during pregnancy can raise maternal blood pressure and interfere with fetal development, so this buffer matters. Progesterone contributes to this same protective mechanism.
On the symptom side, rising estrogen is thought to contribute to skin changes, nasal congestion, and increased blood volume. It may also play a supporting role in nausea, though the evidence linking estrogen directly to morning sickness is weaker than it is for hCG.
Human Placental Lactogen: Redirecting Your Fuel Supply
Human placental lactogen (hPL) is a less well-known hormone, but it does something essential: it rewires your metabolism to prioritize the baby’s nutrition. As hPL levels rise in later pregnancy, it makes your cells more resistant to insulin. This means your body uses less glucose for itself, leaving more available to cross the placenta and fuel fetal growth. It also promotes the breakdown of fat stores, giving your body an alternative energy source while glucose gets redirected.
Most endocrine references describe hPL as a “diabetogenic” hormone because of these insulin-opposing effects. At the same time, hPL stimulates the pancreas to produce more insulin, compensating for the increased resistance. When this balancing act tips too far, the result can be gestational diabetes. The significant rise in insulin resistance during late pregnancy, paralleled by a compensatory increase in insulin production, is largely driven by hPL working alongside other placental hormones.
Relaxin: Loosening Joints for Delivery
Relaxin does exactly what its name suggests. Produced by the ovaries and placenta, it loosens the muscles, joints, and ligaments in your pelvis to prepare your body for childbirth. It also softens and helps expand the cervix before delivery, making it easier for the baby to descend through the birth canal.
Relaxin doesn’t limit its effects to the pelvis, though. It circulates throughout the body and affects connective tissue everywhere, which is why many pregnant people experience joint looseness, lower back pain, or a feeling of instability in the hips and knees. These effects can start well before the third trimester and sometimes linger after delivery.
Oxytocin: Driving Labor and Breastfeeding
Oxytocin operates on a feedback loop that makes it uniquely powerful during labor. When the baby’s head presses against the cervix, nerve signals travel to the brain and trigger the pituitary gland to release oxytocin. That oxytocin stimulates uterine contractions, which push the baby further against the cervix, which triggers more oxytocin. This cycle builds in both intensity and frequency until the baby is delivered.
After birth, oxytocin takes on a second role. When your baby sucks at the breast, the stimulation causes your pituitary gland to release oxytocin, which triggers tiny muscles around the milk-producing glands to contract and push milk through the breast tissue. This is the “let-down” reflex. As long as the baby keeps sucking, your body keeps releasing oxytocin. Once feeding stops, the release stops too, until the next session.
Prolactin: Building the Milk Supply
Prolactin rises throughout pregnancy and works alongside estrogen and progesterone to develop the mammary glands. Specifically, these three hormones together build the mammary alveoli, which are small pockets within the breast that produce and store milk. This development is why breasts enlarge during pregnancy, often well before milk production actually begins.
Even though prolactin is present in high amounts during pregnancy, actual milk production is held in check by progesterone. It’s only after delivery, when progesterone drops sharply, that prolactin can fully activate milk production. Continued breastfeeding keeps prolactin levels elevated, which sustains the milk supply.
The Postpartum Hormonal Shift
After delivery, when the placenta detaches and is expelled, the hormones it was producing collapse rapidly. Estrogen and progesterone, which had been climbing for months, drop sharply within hours to days. This is one of the most dramatic hormonal shifts the human body experiences, and the speed of the decline varies significantly between individuals.
This sudden withdrawal is a major factor in the “baby blues” that affect a large percentage of new parents in the first two weeks after birth. For some, the adjustment is mild and temporary. For others, particularly those with more pronounced hormonal shifts, it can contribute to postpartum depression. Meanwhile, prolactin stays elevated if you’re breastfeeding, and oxytocin continues to pulse with each feeding session, both of which support bonding and milk production in the weeks and months that follow.