Pregnancy hormones are a group of chemical messengers that your body produces in dramatically higher amounts (or for the first time) once an embryo implants in the uterus. They keep the pregnancy viable, reshape your metabolism to feed a growing baby, and physically prepare your body for labor and breastfeeding. At least half a dozen hormones play major roles, and their shifting levels are behind many of the physical and emotional changes you feel from the first weeks through delivery.
hCG: The Hormone Behind the Positive Test
Human chorionic gonadotropin, or hCG, is the hormone home pregnancy tests detect. It’s produced by cells that eventually form the placenta, and it appears in your blood as early as 3 to 4 days after the embryo implants in the uterine wall. Urine tests pick it up about 1 to 2 weeks later, which is why most are accurate around the time of a missed period.
hCG levels rise steeply during the first trimester. At week 3 of pregnancy, blood levels range from roughly 5 to 72 mIU/mL. By week 8, that range jumps to about 31,000 to 149,000 mIU/mL. Levels peak somewhere around weeks 8 through 10, then gradually decline for the rest of the pregnancy as the placenta takes over hormone production on its own.
This rapid rise is also linked to morning sickness. People with especially high hCG levels, including those carrying twins or multiples, tend to experience more severe nausea. A condition called hyperemesis gravidarum, which involves intense, persistent vomiting, is associated with hCG levels that are higher than average. For most people, nausea improves in the second trimester as hCG levels plateau and drop.
Progesterone: Maintaining the Pregnancy
Progesterone is sometimes called the “pregnancy-sustaining hormone,” and for good reason. Early on, the ovaries produce it; later, the placenta takes over. Its primary job is to thicken and maintain the uterine lining so the embryo can implant and grow. It also suppresses uterine contractions, which helps prevent preterm labor.
Beyond the uterus, progesterone affects almost every system in your body. It relaxes smooth muscle tissue throughout the digestive tract, which is why bloating, constipation, and acid reflux are common pregnancy complaints. It also contributes to breast tenderness, fatigue, and mood shifts, particularly in the first trimester when levels are climbing fast. Progesterone stays elevated throughout pregnancy and only drops sharply after delivery, a change that plays a role in postpartum mood fluctuations.
Estrogen: Supporting Growth and Blood Flow
Estrogen levels rise steadily throughout pregnancy and reach concentrations far higher than at any other point in your life. The placenta becomes the main production site, releasing several forms of estrogen that serve overlapping purposes.
Estrogen stimulates the growth of the uterus itself, helps develop the milk duct system in your breasts, and increases blood volume so more oxygen and nutrients reach the placenta. It also triggers changes in skin pigmentation (the “pregnancy glow” and the dark line that can appear on the abdomen) and can make your gums more sensitive and prone to bleeding. Along with progesterone, rising estrogen is a major driver of the emotional ups and downs many people notice, especially during the first and third trimesters.
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 easier. Production soars early in pregnancy and peaks around the end of the first trimester, then gradually declines before surging one last time just before delivery to help relax the pelvis and widen the cervix.
The downside of all that loosening is joint instability. Many pregnant people develop pelvic girdle pain, felt most often at the front of the pubic bone at hip level or in the lower back. Relaxin also affects joints beyond the pelvis, which is why your knees, ankles, and feet may feel less stable than usual. This is one reason healthcare providers often recommend low-impact exercise during pregnancy and caution against sudden, high-force movements.
Human Placental Lactogen: Redirecting Your Metabolism
Human placental lactogen, or hPL, is less well known but plays a critical role in making sure the baby gets enough fuel. It works by increasing insulin resistance in the mother’s body, which means your cells absorb less glucose from the bloodstream. That extra circulating glucose crosses the placenta and feeds the fetus instead.
hPL also ramps up the breakdown of stored fat, releasing fatty acids into your bloodstream so your body can burn fat for energy while sparing glucose and amino acids for the baby. This metabolic shift is one of the main reasons pregnant people sometimes feel hungrier, especially in the second and third trimesters. It’s also why pregnancy increases the risk of gestational diabetes: in some people, the insulin resistance triggered by hPL (along with progesterone and cortisol) overwhelms the body’s ability to compensate, and blood sugar stays too high.
Oxytocin: Driving Labor and Breastfeeding
Oxytocin is produced in the brain and released from the pituitary gland. During pregnancy, it stays relatively quiet until labor begins. Once contractions start, oxytocin drives them forward in a positive feedback loop: contractions stimulate more oxytocin release, which produces stronger contractions, which triggers even more oxytocin, and the cycle continues until delivery.
After birth, oxytocin takes on a second role. It causes tiny muscles around the milk-producing glands in the breast to contract, pushing milk toward the nipple. This is the “let-down reflex” that breastfeeding parents feel, often triggered by a baby’s cry or suckling. Oxytocin is also associated with the intense bonding feelings many parents experience in the hours and days after delivery.
How These Hormones Shift by Trimester
The first trimester is the most hormonally volatile period. hCG doubles roughly every 48 to 72 hours, progesterone and estrogen are climbing, and relaxin hits its first peak. This is when nausea, fatigue, breast tenderness, and mood swings tend to be strongest.
In the second trimester, hCG levels drop and the placenta takes over most hormone production. Many people feel a reprieve from early symptoms during this stretch. Estrogen and progesterone continue rising steadily, and hPL levels increase enough to noticeably shift metabolism, which often brings a bigger appetite.
The third trimester brings the highest overall hormone levels of the entire pregnancy. Estrogen and progesterone peak, relaxin surges again as the body prepares for delivery, and oxytocin receptors in the uterus multiply so contractions can begin effectively when the time comes. Fatigue, joint pain, heartburn, and emotional sensitivity often return or intensify during these final weeks as the hormonal load reaches its maximum.
Cortisol and Prolactin
Two additional hormones deserve mention. Cortisol, the body’s main stress hormone, rises two to three times above normal levels by the third trimester. It helps the baby’s lungs mature and supports the metabolic changes needed to sustain a pregnancy, but it can also contribute to sleep disruption, anxiety, and the “brain fog” many pregnant people describe.
Prolactin prepares the breasts for milk production throughout pregnancy, with levels climbing steadily from the first trimester onward. Despite high prolactin levels, full milk production is held in check by progesterone until after delivery. Once the placenta is delivered and progesterone drops, prolactin takes over and milk production ramps up within a few days.