The main pregnancy hormones are hCG, progesterone, estrogen, relaxin, human placental lactogen, oxytocin, and prolactin. Each one plays a distinct role at different stages, from keeping the pregnancy viable in the first weeks to triggering labor and enabling breastfeeding. Understanding what these hormones actually do helps explain many of the physical changes you experience throughout pregnancy and after delivery.
hCG: The Hormone Behind the Positive Test
Human chorionic gonadotropin, or hCG, is the hormone pregnancy tests detect. Your body starts producing it shortly after a fertilized egg implants in the uterus, and its levels rise rapidly during the first trimester. At four weeks of pregnancy, hCG levels typically range from 0 to 750 units per liter. By weeks 8 through 12, they can climb to between 32,000 and 210,000 units per liter.
The early job of hCG is to support a small structure on the ovary called the corpus luteum, which produces progesterone. Without hCG signaling the corpus luteum to keep working, progesterone levels would drop and the pregnancy couldn’t continue. Rising hCG levels are also thought to be a key driver of morning sickness. Nausea tends to peak when hCG is at its highest, typically during the first trimester, which is why many people feel better as they enter the second trimester and hCG levels plateau or decline.
Progesterone: Protecting the Pregnancy
Progesterone is arguably the most critical hormone for sustaining a pregnancy. In the early weeks, the corpus luteum on the ovary produces it. Around week 12, the placenta takes over production and continues supplying progesterone for the rest of the pregnancy.
Its primary job is to prevent the uterus from contracting too early. Progesterone keeps the uterine muscles relaxed, protecting the fetus from preterm birth. It also suppresses the hormones that trigger ovulation (FSH and LH), ensuring your body doesn’t try to release another egg while you’re already pregnant. The fatigue, bloating, and mood changes many people notice in the first trimester are partly driven by progesterone’s sedating effects on the body.
Estrogen: Supporting Fetal Development
Estrogen levels rise steadily throughout pregnancy and reach concentrations far higher than at any other point in your life. Like progesterone, estrogen is initially produced by the corpus luteum and later by the placenta.
While progesterone focuses on keeping the uterus calm, estrogen is more involved in growing and maturing fetal organs. It promotes the development of fetal tissues and is particularly important for lung and liver maturation. Estrogen does this partly by regulating progesterone production and by triggering the fetus to produce cortisol, a hormone that helps the lungs, liver, thyroid, and adrenal glands mature properly. On the maternal side, estrogen increases blood flow to the uterus, contributes to breast tissue changes, and is partly responsible for the “pregnancy glow” some people experience due to increased circulation to the skin.
Relaxin: Loosening Joints and Ligaments
Relaxin does exactly what its name suggests. It loosens the muscles, joints, and ligaments around your pelvis, back, and abdomen to help your body accommodate a growing baby and eventually make labor and delivery easier. Relaxin levels peak at around 12 to 14 weeks of pregnancy, which is why some people notice pelvic or lower back discomfort surprisingly early.
Beyond the musculoskeletal system, relaxin also affects your cardiovascular system. It helps relax blood vessels and promotes the growth of new ones, allowing your body to handle the significant increase in blood volume that pregnancy demands. This vascular effect is one reason some people experience dizziness or lightheadedness during the first trimester, when relaxin is at its highest. The joint loosening can also make you feel less stable on your feet or more prone to injury during exercise, something worth keeping in mind if you stay active during pregnancy.
Human Placental Lactogen: Redirecting Fuel
Human placental lactogen, or hPL, is produced by the placenta and has a straightforward but important job: it adjusts your metabolism so more nutrients reach the fetus. One way it does this is by making your body less sensitive to insulin. Insulin normally moves sugar from your bloodstream into your cells. When hPL reduces that sensitivity, more glucose stays in the bloodstream and is available for the developing baby instead.
This metabolic shift is why pregnant people sometimes develop gestational diabetes. If your body can’t produce enough extra insulin to compensate for hPL’s effects, blood sugar levels stay too high. It’s also part of why you may feel hungrier as pregnancy progresses: your own cells are getting less of the available glucose, so your body signals you to eat more.
Oxytocin and Prolactin: Labor and Beyond
Oxytocin plays a relatively quiet role for most of pregnancy, then becomes central during labor and breastfeeding. It triggers the strong, rhythmic uterine contractions that push the baby through the birth canal. After delivery, oxytocin continues to cause small uterine contractions that help the uterus shrink back toward its pre-pregnancy size.
During breastfeeding, oxytocin drives what’s known as the let-down reflex. When a baby sucks at the breast, the pituitary gland releases oxytocin, which causes tiny muscles in the breast tissue to contract and push milk out. As long as the baby keeps feeding, oxytocin keeps flowing. Once the baby stops, the release stops until the next feeding session.
Prolactin works alongside oxytocin but focuses on milk production rather than delivery. During pregnancy, prolactin triggers the development of mammary glands and the small pockets within them that actually make and store milk. Despite these physical changes happening months before birth, high levels of estrogen and progesterone during pregnancy block prolactin from initiating full milk production. Milk supply only ramps up after delivery, when estrogen and progesterone drop sharply.
The Postpartum Hormone Shift
Within the first 48 hours after delivery, hCG, estrogen, and progesterone levels all drop dramatically. This is one of the most abrupt hormonal shifts the human body experiences. The sudden withdrawal of estrogen and progesterone, hormones that were circulating at extremely high levels for months, is a major contributor to the mood swings, tearfulness, and anxiety commonly called the “baby blues.”
Prolactin and oxytocin remain elevated if you breastfeed, which is why breastfeeding can feel calming for some people. For those who don’t breastfeed, prolactin levels gradually decline over the weeks following delivery. The full hormonal rebalancing process varies from person to person, but most hormones return close to pre-pregnancy levels within a few months, though the timeline can be longer for those who continue to nurse.