At least eight hormones rise significantly during pregnancy, each playing a distinct role in supporting the growing baby and reshaping the mother’s body. Some peak early and taper off, while others climb steadily until delivery. Here’s what each one does and when it matters most.
hCG: The Hormone That Confirms Pregnancy
Human chorionic gonadotropin, or hCG, is the hormone detected by pregnancy tests. It rises rapidly in the first weeks after conception and peaks toward the end of the first trimester, then gradually decreases through the rest of pregnancy. Its primary job is to keep the corpus luteum (a temporary structure in the ovary) alive so it can continue producing progesterone, the hormone that sustains the pregnancy in its earliest stages. Without hCG, the corpus luteum would break down and progesterone levels would drop, ending the pregnancy before the placenta could take over.
Progesterone: Keeping the Uterus Calm
Progesterone is arguably the most important hormone of pregnancy. Its central function is to maintain a quiet, stable environment for the developing baby by reducing the force and frequency of uterine contractions. In the first weeks, the corpus luteum produces nearly all of it. Around the seventh gestational week, the placenta begins making substantial amounts of progesterone on its own, and by midpregnancy the placenta has fully taken over production in a process called the luteoplacental shift.
Progesterone’s effects reach well beyond the uterus. It relaxes the smooth muscle lining blood vessels, which lowers blood pressure and improves blood flow to the placenta. It also slows the digestive tract by directly inhibiting the contractile activity of stomach and intestinal muscles. This is why constipation, bloating, and slower digestion are so common during pregnancy. The hormone is essentially telling smooth muscle throughout the body to ease up, and the digestive system gets caught in the crossfire.
Estrogen: Fueling Growth and Preparing for Delivery
Three types of estrogen circulate during pregnancy, with estriol being the dominant form produced by the placenta. Estrogen levels climb continuously and reach their highest point just before delivery. The hormone helps the uterus expand as the baby grows larger, makes the body more responsive to other pregnancy hormones, and prepares the breasts and birth canal for labor, delivery, and breastfeeding. Estrogen also increases blood flow to the uterus and plays a role in fetal organ development.
Relaxin: Loosening Joints and Ligaments
Relaxin does exactly what its name suggests. It loosens the muscles and ligaments in the pelvis to prepare the body for delivery. Levels peak at around 12 to 14 weeks of pregnancy, which is earlier than many people expect. This early peak explains why joint pain, pelvic discomfort, and a feeling of “looseness” in the hips can start well before the belly is visibly large. Relaxin doesn’t target the pelvis exclusively. It affects connective tissue throughout the body, which is why some pregnant people notice increased flexibility or instability in their knees, ankles, and lower back.
Prolactin: Preparing the Breasts for Milk
Prolactin levels increase roughly tenfold during pregnancy, rising from about 20 to 200 nanograms per milliliter by the third trimester. This hormone drives the growth and development of breast tissue so it’s ready to produce milk after delivery. Despite the high levels, actual milk production is held in check by progesterone and estrogen until the placenta is delivered and those hormones drop sharply. That sudden hormonal shift is what triggers the milk to “come in” a few days after birth.
Human Placental Lactogen: Redirecting Fuel to the Baby
Human placental lactogen, or hPL, is produced by the placenta and steadily increases as the placenta grows. Its main role is to reshape how the mother’s body uses energy. hPL increases insulin resistance, which means the mother’s cells absorb less glucose from the bloodstream. The result is higher circulating blood sugar, and that extra glucose crosses the placenta to feed the baby.
hPL also ramps up the breakdown of stored fat, releasing fatty acids that the mother’s body can burn as fuel instead of glucose. During periods of fasting (even overnight sleep), this mechanism spares glucose and amino acids for the baby. The trade-off is that this insulin resistance can tip into gestational diabetes in some pregnancies, particularly when combined with the effects of progesterone and cortisol, which also reduce insulin sensitivity.
Thyroid Hormones: Supporting Fetal Brain Development
The thyroid gland works considerably harder during pregnancy, increasing 10 to 40 percent in size. Production of its two main hormones surges by about 50 percent to meet the demands of both mother and baby. Early in pregnancy, before the fetal thyroid is functional, the baby depends entirely on the mother’s supply. These hormones are critical for fetal brain development and nervous system formation. The increased demand is why iodine requirements go up during pregnancy and why thyroid function is closely monitored, especially in people with pre-existing thyroid conditions.
Melanocyte-Stimulating Hormone: Changes in Skin Pigmentation
Melanocyte-stimulating hormone (MSH) levels rise during pregnancy and directly stimulate the cells that produce skin pigment. This is responsible for several recognizable skin changes: the linea nigra (a dark line running down the center of the belly), darkening of the areolas, and melasma, the patchy brown discoloration that often appears on the face. These changes are more pronounced in people with darker skin tones and typically fade after delivery, though melasma can sometimes persist for months or longer.
How These Hormones Work Together
These hormones don’t operate in isolation. hCG sustains progesterone production until the placenta is mature enough to take over. Estrogen amplifies the body’s sensitivity to other pregnancy hormones, making their effects more potent. hPL and progesterone both contribute to insulin resistance, compounding each other’s metabolic effects. Prolactin prepares the breasts, but estrogen and progesterone block actual milk release until after delivery.
Many common pregnancy symptoms trace directly back to this hormonal cascade. Morning sickness correlates with the hCG peak in the first trimester. Constipation and heartburn come from progesterone slowing the gut. Pelvic and back pain link to relaxin loosening connective tissue. Skin darkening follows the rise in MSH. Understanding which hormone is behind a particular symptom can make the experience feel less random, even when it doesn’t make it more comfortable.