Several hormones increase during pregnancy, but the one most closely associated with it is human chorionic gonadotropin, or hCG. This is the hormone that pregnancy tests detect, and it can rise from virtually zero to over 100,000 mIU/mL within the first two months. Beyond hCG, pregnancy triggers significant increases in progesterone, estrogen, relaxin, prolactin, cortisol, and several other hormones that each play a distinct role in supporting the pregnancy and preparing your body for delivery.
hCG: The Pregnancy Hormone
Human chorionic gonadotropin is produced almost immediately after a fertilized egg implants in the uterus. It’s what makes a home pregnancy test turn positive. At week 3 of gestation, hCG levels typically range from 6 to 71 mIU/mL. By week 8, they can reach 32,065 to 149,571 mIU/mL.
In early pregnancy, hCG roughly doubles every 48 hours. The minimum expected rise over two days is about 49% when levels are below 1,500 mIU/mL, 40% when levels are between 1,500 and 3,000, and 33% once they climb above 3,000. A rise of at least 35% over two days is generally considered a sign that a pregnancy is progressing normally. Levels peak around weeks 8 to 12 and then gradually decline, settling into a lower range for the remainder of the pregnancy. By week 17, for example, levels typically sit between about 8,175 and 55,868 mIU/mL.
The primary job of hCG is to signal the ovaries to keep producing progesterone during the early weeks, before the placenta is mature enough to take over that role. It’s also the hormone most associated with morning sickness, which is why nausea tends to be worst during the first trimester when hCG peaks.
Progesterone: Maintaining the Pregnancy
Progesterone is the hormone that keeps the uterine lining thick and blood-rich so the embryo can implant and grow. In the first trimester, progesterone levels range from about 10 to 44 ng/mL. By the second trimester, they climb to 19.5 to 82.5 ng/mL, and in the third trimester they can reach 65 to 290 ng/mL.
Early in pregnancy, the ovaries produce progesterone in response to hCG. Around weeks 8 to 12, the placenta takes over as the primary source. Progesterone also relaxes smooth muscle throughout the body, which helps prevent the uterus from contracting too early. That same muscle-relaxing effect is responsible for some of the less welcome symptoms of pregnancy: slower digestion, bloating, and constipation.
Estrogen and Its Wide-Ranging Effects
Estrogen levels rise steadily throughout pregnancy and reach concentrations far higher than at any other point in a woman’s life. The placenta becomes the main source, producing increasing amounts as it grows. Estrogen promotes blood flow to the uterus, helps the mammary glands develop for breastfeeding, and stimulates the production of other hormones the pregnancy depends on.
One of estrogen’s less obvious roles is driving up cortisol-binding globulin, a protein that carries the stress hormone cortisol in the blood. This is one reason total cortisol levels increase two to three times their normal concentration during pregnancy. Free cortisol rises too, which helps mobilize energy and supports the immune system adjustments that allow the body to tolerate the growing fetus.
Relaxin: Loosening Joints and Ligaments
Relaxin does exactly what its name suggests. It loosens the muscles, joints, and ligaments of the pelvis to prepare for delivery. Levels peak around 12 to 14 weeks of pregnancy, near the end of the first trimester. A second surge occurs just before labor, helping the cervix soften and widen.
Because relaxin doesn’t only target the pelvis, it can make joints throughout the body feel looser and less stable. This is a common reason pregnant women experience lower back pain, hip discomfort, or a feeling of “wobbliness” in their joints, especially as the pregnancy progresses and body weight shifts forward.
Prolactin: Preparing for Breastfeeding
Prolactin starts rising around the fifth week of pregnancy and eventually reaches 10 to 20 times its pre-pregnancy concentration. Its main job is preparing breast tissue to produce milk. Despite these high levels, actual milk production is suppressed during pregnancy by the high levels of progesterone and estrogen. Once the placenta is delivered and those hormone levels drop sharply, prolactin takes over and milk production begins.
Human Placental Lactogen: Fueling the Fetus
Human placental lactogen, or hPL, is produced by the placenta and plays a key role in how your body handles energy during pregnancy. It makes your cells less sensitive to insulin, the hormone that normally pulls sugar out of the bloodstream and into cells. With reduced insulin sensitivity, more glucose stays in the bloodstream and is available to cross the placenta and fuel fetal growth.
This shift in metabolism is normal, but when it becomes too pronounced, it can contribute to gestational diabetes. hPL levels rise as the placenta grows, which is why gestational diabetes screening typically happens in the second trimester, when the metabolic shift is significant enough to test for.
Thyroid Hormones
The thyroid gland works harder during pregnancy to meet the metabolic demands of both the mother and the developing baby. In the first trimester, hCG directly stimulates the thyroid, which is why TSH (the signal from the brain that tells the thyroid to produce more hormone) tends to dip early on. Normal TSH in the first trimester runs roughly 0.49 to 2.91 mIU/L, lower than typical non-pregnant ranges. By the second trimester, TSH rises to about 0.73 to 4.22 mIU/L as hCG levels fall and the thyroid settles into a new equilibrium.
Free T4, the active thyroid hormone circulating in the blood, is slightly higher in the first trimester and then decreases modestly in the second. These shifts matter because untreated thyroid problems during pregnancy can affect fetal brain development, which is why thyroid function is often checked early in prenatal care.
Oxytocin: The Labor Trigger
Oxytocin is best known for triggering contractions during labor. While levels don’t spike dramatically for most of pregnancy, something important happens as labor nears: the uterine muscle starts producing more oxytocin receptors. This means the uterus becomes increasingly sensitive to even normal amounts of oxytocin, which is how the body transitions from months of keeping the uterus quiet to the powerful, rhythmic contractions of labor. After delivery, oxytocin continues to play a role by stimulating the “let-down” reflex during breastfeeding.
Skin Changes and Melanocyte-Stimulating Hormone
Many pregnant women notice their skin darkening in certain areas. A dark line running from the pubic bone up the center of the abdomen, called the linea nigra, commonly appears in late pregnancy. Darkening of the nipples and patches of discoloration on the face (melasma) are also common. These changes are driven by melanocyte-stimulating hormone, which the placenta produces in increasing amounts. This hormone activates the pigment-producing cells in the skin, and the effect is most visible in areas where those cells are naturally concentrated.
These pigmentation changes typically fade in the months after delivery, though melasma can sometimes persist, particularly with sun exposure.