Pregnancy doesn’t just cause hormonal imbalance. It is one of the most dramatic hormonal shifts the human body ever undergoes. From the moment of implantation, the placenta begins producing hormones that reshape nearly every system in your body, from your metabolism and mood to your thyroid, joints, and skin. Most of these changes are normal and necessary, but they can sometimes tip into territory that causes real problems.
The Major Hormones That Surge During Pregnancy
Three hormones drive most of what you feel during pregnancy: human chorionic gonadotropin (hCG), progesterone, and estrogen. Each follows its own timeline and serves a distinct purpose.
hCG is produced almost exclusively by the placenta and rises sharply during the first trimester. It’s the hormone detected by pregnancy tests, and it’s also a likely contributor to first-trimester nausea and vomiting. Levels climb rapidly in early weeks, peak around weeks 8 to 11, then gradually taper.
Progesterone is the most abundant hormone in pregnancy. Early on, the ovaries produce it; later, the placenta takes over. Its primary job is thickening the uterine lining and maintaining the environment the embryo needs to implant and grow. Progesterone also has a calming effect on the brain. One of its breakdown products enhances the activity of your brain’s main inhibitory signaling system, which is why some women feel unusually relaxed or sedated during parts of pregnancy.
Estrogen, normally produced by the ovaries, is also manufactured by the placenta during pregnancy. It supports blood flow to the uterus, helps the fetal organs develop, and prepares breast tissue for milk production. Levels climb steadily across all three trimesters.
How Pregnancy Changes Your Thyroid
Your thyroid gland works harder during pregnancy to meet the metabolic demands of both your body and the developing fetus. hCG structurally resembles thyroid-stimulating hormone (TSH), so rising hCG levels in the first trimester can actually stimulate the thyroid and suppress TSH. This is why the normal reference range for TSH shifts during pregnancy. The Endocrine Society recommends keeping TSH between 0.2 and 2.5 mU/L in the first trimester, and between 0.3 and 3.0 mU/L in the second and third trimesters, both lower than the standard non-pregnant range.
If your thyroid can’t keep up with these demands, pregnancy can unmask or worsen hypothyroidism. On the other end, the hCG-driven stimulation can temporarily push thyroid function too high, sometimes causing a condition called gestational thyrotoxicosis. Either direction can affect miscarriage risk, fetal development, and how you feel day to day, which is why thyroid screening is part of routine prenatal care for many providers.
Cortisol Rises Steadily Throughout Pregnancy
Cortisol, your body’s primary stress hormone, follows an exponential growth curve during pregnancy. One study tracking cortisol across gestation found levels averaged about 44.2 μg/dL in the first gestational week, then increased roughly 2% per week from that point forward. The steepest rise happens in the second and third trimesters, driven by both estrogen and a hormone produced by the placenta called corticotropin-releasing hormone (CRH).
This isn’t a sign that something is wrong. Rising cortisol supports fetal organ maturation, particularly lung development, and helps regulate the immune adaptations that prevent your body from rejecting the pregnancy. But the sustained elevation can contribute to symptoms that feel a lot like chronic stress: disrupted sleep, anxiety, elevated blood sugar, and changes in appetite. For women who already have anxiety or mood disorders, the cortisol climb can intensify those experiences.
Insulin Resistance and Gestational Diabetes
In late pregnancy, your body deliberately becomes more resistant to insulin. This is a feature, not a bug. The placenta produces a hormone called human placental lactogen (hPL) that works against insulin, redirecting glucose and amino acids across the placenta to fuel fetal growth. Most endocrine references describe hPL as a “diabetogenic” hormone because of its insulin-antagonistic effects. It peaks after overnight fasting, precisely when your glucose and insulin levels are at their lowest, ensuring the fetus still gets nutrients even when you haven’t eaten.
Normally, your pancreas compensates by ramping up insulin production. hPL actually plays a role here too, stimulating the growth of insulin-producing cells and boosting their output. But if your pancreas can’t keep pace with the increasing resistance, blood sugar climbs too high and you develop gestational diabetes. Screening typically happens between weeks 24 and 28 with a glucose tolerance test. Under widely used criteria, a fasting blood sugar of 92 mg/dL or higher, a one-hour reading of 180 mg/dL or higher, or a two-hour reading of 153 mg/dL or higher on the test is enough for a diagnosis.
Joint Looseness and Pelvic Pain
Starting around weeks 10 to 12, the hormone relaxin begins reshaping your connective tissue. Produced first by the ovaries and then the placenta, relaxin alters the structure of collagen in your ligaments, making them more flexible. This is essential for delivery: the pelvic joints need to loosen enough for the baby to pass through. Relaxin levels rise to a considerable level during the first trimester, plateau through mid-pregnancy, and become undetectable within days of delivery.
The trade-off is that this looseness isn’t limited to your pelvis. Relaxin circulates throughout your body, which is why many pregnant women experience lower back pain, hip instability, or a feeling that their joints are “wobbly.” In some cases, the pubic symphysis (the joint at the front of the pelvis) separates enough to cause significant pain with walking, climbing stairs, or rolling over in bed. This is called pelvic girdle pain, and it’s a direct consequence of how relaxin changes collagen.
Hair, Skin, and Other Visible Changes
The hormonal surge during pregnancy extends your hair’s growth phase. Normally, a percentage of your hair follicles cycle into a resting phase (telogen) and eventually shed. During pregnancy, elevated estrogen and progesterone keep more follicles in the active growth phase (anagen) for longer. Research comparing hair samples during and after pregnancy found that the anagen rate increases significantly during the sixth and ninth months of pregnancy.
The flipside comes after delivery. When hormone levels drop, all those follicles that stayed in the growth phase shift to the resting phase at once. Within three to six months postpartum, this triggers a wave of shedding called telogen effluvium. It can look alarming, with clumps of hair coming out in the shower, but it’s temporary and reflects the follicles catching up on a normal cycle that was delayed by pregnancy hormones. For most women, hair density returns to its pre-pregnancy baseline.
The Postpartum Hormone Crash
The most abrupt hormonal shift doesn’t happen during pregnancy. It happens immediately after delivery. When the placenta is delivered, your body loses its largest hormone-producing organ in a matter of minutes. Progesterone, which was the most abundant hormone in your body just hours earlier, plummets. Estrogen drops sharply as well, and the body adapts to low estrogen levels to support lactation.
This sudden withdrawal creates a hormonal environment that is essentially the opposite of what your brain adapted to over nine months. Prolactin surges to support breastfeeding, but estrogen and progesterone are nearly absent. Dopamine levels may dip. Thyroid function can fluctuate. The combination of high prolactin, absent progesterone, low estrogen, and thyroid variability produces a significant psychological impact for many women. Research in animal models has shown that artificially inducing drops in ovarian hormones triggers depressive behavior, which supports the idea that the postpartum hormone crash contributes directly to mood disturbances and, in some cases, postpartum depression.
The “baby blues,” affecting up to 80% of new mothers in the first two weeks, reflect this adjustment period. Postpartum depression, which is more severe and longer lasting, likely involves a combination of this hormonal withdrawal, sleep deprivation, and individual differences in how the brain responds to the loss of progesterone’s calming metabolites.
When Normal Shifts Become Problematic
The distinction between a normal pregnancy hormonal shift and a problematic imbalance isn’t always obvious, because pregnancy changes the definition of “normal.” TSH levels that would be flagged outside of pregnancy are expected during the first trimester. Cortisol levels that would suggest a disorder in a non-pregnant person are routine by the third trimester. Insulin resistance that would be treated as prediabetes in other contexts is a built-in feature of late gestation.
The shifts that cross into genuine imbalance are the ones that exceed what the body can compensate for. Gestational diabetes happens when insulin resistance outpaces pancreatic output. Gestational thyroid disorders happen when the thyroid can’t match the increased demand. Severe mood changes happen when the brain’s response to rapidly shifting neurosteroids overwhelms its coping mechanisms. These aren’t separate conditions from pregnancy. They’re the far end of the same hormonal spectrum every pregnant person navigates.