Pregnancy triggers the most dramatic hormonal shift the human body ever experiences. From the moment an embryo implants, dozens of hormones rise, fall, and interact in ways that sustain the pregnancy, reshape your metabolism, and prepare your body for birth and breastfeeding. These changes follow a roughly predictable timeline, with each trimester bringing distinct hormonal priorities.
The First Trimester: hCG Takes Center Stage
The earliest hormonal signal of pregnancy is human chorionic gonadotropin, or hCG. This is the hormone detected by pregnancy tests. It’s produced by the cells that will become the placenta, and its job in the first weeks is to keep the ovaries producing progesterone, which maintains the uterine lining and prevents a period from starting.
hCG rises fast. In a healthy early pregnancy, levels increase by at least 35 to 49 percent every 48 hours. At week 3 (roughly the time of implantation), hCG typically ranges from 6 to 71 mIU/mL. By week 8, it can reach 32,000 to nearly 150,000. That explosive climb peaks somewhere between weeks 8 and 12, then gradually declines and plateaus for the rest of the pregnancy. The rapid rise of hCG is strongly linked to first-trimester nausea: the faster and higher it climbs, the worse morning sickness tends to be.
Progesterone, meanwhile, is doing essential structural work. Produced first by the ovaries and then increasingly by the placenta (which takes over around weeks 10 to 12), progesterone relaxes smooth muscle tissue throughout the body. That includes the uterus, preventing contractions, but also the digestive tract, which is why bloating, constipation, and heartburn often appear early on.
Estrogen’s Enormous Rise
Estrogen increases throughout all three trimesters, but the scale of that increase is staggering. Before pregnancy, estradiol (the primary form of estrogen) typically ranges from 30 to 400 pg/mL depending on where you are in your menstrual cycle. By the third trimester, levels can exceed 30,000 pg/mL. That’s roughly 100 to 1,000 times higher than non-pregnant levels.
This surge serves multiple purposes. Estrogen promotes blood vessel growth in the uterus and placenta, stimulates breast tissue development, and helps regulate other hormones. It also increases uterine sensitivity to oxytocin as the due date approaches, essentially priming the uterus for labor contractions. Near the end of pregnancy, high circulating estrogen makes oxytocin receptors on the uterine wall more responsive, which is part of why labor can begin on its own.
Relaxin and Your Joints
A lesser-known hormone called relaxin peaks early, around 12 to 14 weeks. Its name describes exactly what it does: it loosens the muscles, joints, and ligaments of the pelvis, back, and abdomen. This loosening eventually helps accommodate a growing uterus and makes delivery physically possible, but it begins months before it’s needed.
The trade-off is that relaxin doesn’t target only your pelvis. It affects connective tissue throughout the body, which is why many pregnant people experience joint instability, lower back pain, or a feeling of “looseness” in their hips and knees well before the third trimester. These effects can linger for months postpartum as relaxin levels slowly return to baseline.
Metabolic Hormones and Blood Sugar
Starting in the second trimester, the placenta produces a hormone called human placental lactogen (hPL) that fundamentally changes how your body uses energy. hPL reduces how efficiently your cells respond to insulin and decreases how much glucose your own tissues absorb. The result is that more glucose stays in your bloodstream, available to cross the placenta and feed the growing fetus.
This is a deliberate metabolic trade-off. Your body becomes mildly insulin-resistant on purpose, redirecting sugar to the baby. hPL also increases the breakdown of stored fat, releasing fatty acids that your body can burn as an alternative fuel source, further sparing glucose for fetal nutrition. At the same time, hPL stimulates your pancreas to produce more insulin to compensate. When the pancreas can’t keep up with the increased demand, blood sugar rises too high, and that’s when gestational diabetes develops.
Progesterone and cortisol (which also rises significantly during pregnancy) contribute to this insulin resistance as well. It’s not a single-hormone effect but a coordinated shift driven by the placenta.
Thyroid Hormones Work Harder
Your thyroid gland increases its output substantially during pregnancy. The daily requirement for thyroid hormone rises by an estimated 30 to 50 percent, and this increase happens almost entirely in the first trimester. Requirements typically plateau around 16 weeks. The fetus depends on maternal thyroid hormones for brain development in early pregnancy, before its own thyroid gland becomes functional.
For most people, a healthy thyroid gland compensates for this increased demand without any issues. But for those who take thyroid medication, the dosage often needs to increase early in pregnancy. About 85 percent of pregnancies in people on thyroid replacement require a nearly 50 percent dosage increase, predominantly in the first trimester.
Progesterone, Mood, and the Brain
Progesterone doesn’t just act on the uterus. The body converts some of it into a compound called allopregnanolone, which directly affects brain chemistry. Allopregnanolone enhances the activity of GABA, the brain’s primary calming neurotransmitter. It essentially amplifies the brain’s built-in “slow down” signals, which can produce a feeling of calm or sedation during pregnancy.
This sounds straightforwardly positive, but the picture is more complicated. As allopregnanolone levels rise over the course of pregnancy, the brain adapts by changing the structure of its GABA receptors, reducing the number and composition of certain receptor subtypes. Think of it as the brain turning down its own volume knob to compensate for the louder signal. This adaptation is part of why the sudden withdrawal of these hormones after birth can trigger mood disturbances. The brain has recalibrated for a hormonal environment that no longer exists.
Prolactin Rises but Waits
Prolactin, the hormone responsible for milk production, rises steadily throughout pregnancy. By the third trimester it can be 10 to 20 times higher than pre-pregnancy levels. Yet milk production doesn’t begin in earnest until after delivery. The reason is straightforward: the high levels of estrogen and progesterone circulating during pregnancy actively block prolactin from triggering full milk synthesis. The machinery is being assembled, but it’s held in check.
Once the placenta is delivered and estrogen and progesterone plummet, that block is removed. Prolactin, already elevated, can then stimulate the breast tissue to produce milk. This is why the onset of mature milk production (as opposed to colostrum) typically takes two to five days after birth, roughly tracking the timeline of hormonal withdrawal.
The Postpartum Crash
The hormonal transition after delivery is abrupt. The placenta, which had become a major hormone-producing organ, is gone within minutes. The drop in estrogen and progesterone that follows is one of the fastest hormonal shifts the body experiences.
The numbers illustrate this vividly. In one longitudinal study, mean estradiol levels near the end of pregnancy measured around 117 pg/mL on a standardized scale. By the first day postpartum, they had dropped to about 15 pg/mL. By day three, they were below 6. Progesterone followed a similar trajectory: from roughly 1,914 pg/mL near term to 184 on postpartum day one, then down to about 55 by day five. Within less than a week, levels of both hormones had fallen by over 95 percent.
This withdrawal is why the early postpartum days feel so physically and emotionally intense. Night sweats, mood swings, hair changes, and the “baby blues” all correlate with this rapid hormonal recalibration. For most people the adjustment period is temporary, resolving within a few weeks as the body establishes a new hormonal baseline, one now shaped by whether or not breastfeeding is occurring, since nursing maintains elevated prolactin and suppresses the return of estrogen and ovulation.