Several hormones rise dramatically during pregnancy, each playing a specific role in keeping the pregnancy viable and preparing your body for delivery. The major ones include hCG, progesterone, estrogen, relaxin, cortisol, human placental lactogen, and thyroid hormones. Some increase within days of conception, while others climb gradually over months. Together, they reshape nearly every system in your body, which is why pregnancy affects everything from your energy levels to your skin.
Human Chorionic Gonadotropin (hCG)
hCG is the hormone that makes a pregnancy test turn positive. It’s produced by the cells that will become the placenta, and its primary job early on is to signal your ovaries to keep making progesterone so the uterine lining stays intact. Without hCG, the lining would shed and the pregnancy would end.
hCG rises faster than any other pregnancy hormone. At 3 weeks, levels range from about 5 to 72 mIU/mL. By week 7, they can reach over 150,000 mIU/mL. The wide ranges are normal and vary significantly between pregnancies. After peaking somewhere around weeks 8 to 12, hCG declines slightly and levels off for the remainder of the pregnancy.
This rapid early surge is strongly linked to morning sickness. The timing lines up almost exactly: nausea tends to peak when hCG peaks, and it often improves in the second trimester as levels taper off. Not everyone experiences this equally, but the hormone’s steep climb is the most widely accepted explanation for first-trimester nausea.
Progesterone
Progesterone is sometimes called the “pregnancy maintenance hormone,” and for good reason. It thickens the uterine lining to support implantation of the fertilized egg, and then it keeps that lining stable throughout pregnancy. Early on, the ovaries produce most of the progesterone. By around 10 to 12 weeks, the placenta takes over.
Beyond the uterus, progesterone relaxes smooth muscle throughout your body. This is why it contributes to common symptoms like bloating, constipation, and acid reflux: the same muscle-relaxing effect that keeps the uterus calm also slows down digestion. Progesterone is also a likely contributor to first-trimester fatigue, as it has a mild sedative-like effect on the brain. By the third trimester, progesterone levels are roughly 10 times higher than they were before pregnancy.
Estrogen
Estrogen rises steadily throughout pregnancy and reaches levels far beyond what the body normally produces. Initially made by the ovaries, it’s later produced in large quantities by the placenta to help maintain a healthy pregnancy. Estrogen supports blood flow to the uterus, helps the placenta function, and plays a role in the development of fetal organs.
Rising estrogen also drives many visible changes. It stimulates growth of breast tissue and helps prepare the milk ducts for breastfeeding. Combined with progesterone, the increase in estrogen during pregnancy is thought to trigger melasma, the patches of darker skin that commonly appear on the face, according to the American Academy of Dermatology. Estrogen also contributes to nasal congestion (pregnancy rhinitis) by increasing blood flow and swelling mucous membranes.
Relaxin
Relaxin is present at low levels before pregnancy, rising around ovulation each cycle. If conception occurs, production increases sharply and peaks between weeks 12 and 14. After that, relaxin declines through the middle of pregnancy before surging again just before delivery.
Its job is exactly what its name suggests: it loosens things up. Relaxin targets the muscles, joints, and ligaments of the pelvis, making them more flexible to accommodate a growing baby and eventually allow delivery. That final pre-delivery surge helps widen the cervix and soften it for labor. The tradeoff is that this loosening isn’t limited to the pelvis. Relaxin affects joints throughout the body, which is why many pregnant people experience lower back pain, hip instability, or a feeling of general “looseness” in their joints, particularly in the second and third trimesters.
Cortisol
Cortisol, the body’s primary stress hormone, rises gradually throughout pregnancy. By the third trimester, circulating cortisol levels are two to three times higher than normal. This isn’t a sign of distress. The increase serves a specific developmental purpose.
In late pregnancy, a surge in cortisol helps the fetal lungs, heart, brain, and kidneys mature. The lung effect is especially critical: cortisol triggers the production of surfactant, a substance that keeps the lungs’ tiny air sacs from collapsing so the baby can breathe after birth. This is so important that when preterm delivery is expected, doctors give synthetic steroids that mimic this effect to accelerate lung development before birth. The elevated cortisol can, however, contribute to symptoms you feel, including difficulty sleeping, increased anxiety, and higher blood sugar levels in the third trimester.
Human Placental Lactogen (hPL)
Human placental lactogen is produced by the placenta and rises steadily as the placenta grows, reaching its highest levels in the third trimester. Its main role is metabolic: hPL makes your cells less sensitive to insulin, which keeps more glucose circulating in the bloodstream where the fetus can access it. It also helps regulate maternal metabolism more broadly, ensuring a steady supply of nutrients crosses the placenta.
This deliberate reduction in insulin sensitivity is one reason gestational diabetes develops in some pregnancies. When hPL-driven insulin resistance exceeds the body’s ability to compensate by producing more insulin, blood sugar levels climb too high. It’s also why glucose screening typically happens around weeks 24 to 28, when hPL levels are high enough to stress the insulin system.
Thyroid Hormones
Your thyroid gland works harder during pregnancy to meet the metabolic demands of both your body and the developing fetus. Thyroid hormone production increases by roughly 50% over the course of pregnancy. In the first trimester, hCG itself stimulates the thyroid (since hCG is structurally similar to thyroid-stimulating hormone), which can temporarily push free thyroxine levels up and TSH levels down.
Normal TSH reference ranges shift during pregnancy. In the first trimester, TSH typically runs lower than in non-pregnant adults, with a range around 0.5 to 2.9 mIU/L. By the second trimester, it climbs slightly to roughly 0.7 to 4.2 mIU/L. These adjusted ranges matter because thyroid levels that look abnormal by standard reference ranges may actually be perfectly normal for pregnancy. Untreated thyroid imbalances, whether too high or too low, can affect fetal brain development and increase the risk of pregnancy complications, which is why thyroid function is often monitored.
How These Hormones Create Common Symptoms
Most pregnancy symptoms trace back to one or more of these hormonal shifts. First-trimester nausea aligns with the hCG peak. Fatigue in early pregnancy corresponds to rapidly rising progesterone. The skin darkening many people notice on their face, abdomen, or nipples is driven by the combined increase in estrogen and progesterone, which stimulates the cells that produce skin pigment.
Joint pain and pelvic pressure relate directly to relaxin loosening connective tissue. Third-trimester insomnia and anxiety have roots in elevated cortisol. The increased appetite and blood sugar fluctuations of later pregnancy connect to hPL’s effect on insulin sensitivity. Even the feeling of being warmer than usual comes from thyroid hormones boosting your metabolic rate. These symptoms aren’t random. They’re the side effects of a hormonal environment specifically engineered to grow and deliver a baby.