Pregnancy hormones hit their most intense levels during the first trimester, roughly between weeks 8 and 14, when the hormone responsible for sustaining early pregnancy peaks and drives the worst of the nausea, fatigue, and mood swings. But “the worst” depends on which hormone you’re talking about. Different hormones peak at different times, and each one brings its own set of symptoms that can make certain weeks feel harder than others.
Weeks 8 to 14: The First Trimester Peak
The hormone most responsible for early pregnancy misery is human chorionic gonadotropin, or hCG. Your body starts producing it the moment a fertilized egg implants, and levels climb rapidly until they peak between weeks 8 and 11. This surge directly correlates with the timing of the worst nausea and vomiting. Women with higher hCG levels in their blood and urine consistently report more severe symptoms than women with lower levels.
Nausea and vomiting typically peak between weeks 10 and 16 and resolve by week 20 for most women. That overlap with peak hCG is not a coincidence. The body is producing enormous quantities of this hormone to maintain the pregnancy, and the side effects are significant. If you’re in the thick of weeks 8 through 14 and feel like everything is getting worse, that’s because hormonally, it is. The good news is that hCG steadily declines after this peak and levels off for the rest of pregnancy.
This same hCG surge also stimulates your thyroid. The hormone mimics a thyroid-stimulating signal, temporarily increasing thyroid hormone production while suppressing the body’s normal thyroid regulation. For some women, this creates a brief period of feeling jittery, overheated, or anxious on top of the nausea. It’s most pronounced between weeks 9 and 12, then fades as hCG drops.
What Relaxin Does in the First Trimester
Relaxin, a hormone that loosens ligaments and connective tissue, also peaks around weeks 12 to 14. Its job is to prepare your body for the physical changes ahead, but the loosening it causes can trigger pelvic girdle pain surprisingly early. You might feel this as discomfort in the front of your pubic bone, across your lower back, or in your inner thighs. It often shows up when climbing stairs, getting out of a car, or rolling over in bed. Relaxin levels decline after the first trimester before surging again just before delivery, which is why some women notice a brief improvement in joint pain during the second trimester before it returns later.
Progesterone: Fatigue and Digestive Slowdown
Progesterone rises throughout the entire pregnancy, but its effects are often felt most acutely in two phases: the first trimester, when levels jump sharply from your pre-pregnancy baseline, and the third trimester, when levels reach 100 to 200 nanograms per milliliter.
Progesterone’s primary side effect is smooth muscle relaxation. It triggers chemical pathways in muscle cells that reduce their ability to contract. This is essential for preventing the uterus from contracting too early, but it also relaxes the muscles of your digestive tract. The result is slower digestion, bloating, constipation, and acid reflux. These symptoms tend to worsen as progesterone climbs, making the third trimester particularly uncomfortable from a digestive standpoint.
Progesterone also affects energy metabolism. It inhibits glucose uptake and changes how your liver processes sugar, which contributes to the crushing fatigue many women experience. First-trimester exhaustion often feels disproportionate to what’s physically happening because progesterone is essentially sedating your system while your body adapts to a rapid hormonal shift.
The Second Trimester Break
There’s a reason the second trimester is often called the “honeymoon phase.” HCG has dropped from its peak. Relaxin has come down. Progesterone is still rising, but your body has had time to adjust to it. Nausea usually fades by week 20. Energy often returns. This doesn’t mean you’ll feel completely normal, but the hormonal chaos of the first trimester has calmed, and the heaviest physical burden of the third trimester hasn’t arrived yet.
Third Trimester: A Different Kind of Intense
The final weeks of pregnancy bring a second hormonal surge that feels different from the first trimester but can be just as difficult. Estrogen and progesterone both reach their highest concentrations near week 40, with progesterone production hitting roughly 250 milligrams per day by the placenta alone. Estrogen rises to activate the uterus and prepare it for efficient labor. Cortisol levels climb to two to four times their normal range, peaking in the third trimester. This cortisol increase supports fetal lung development, but it also amplifies feelings of stress, anxiety, and emotional volatility.
The combination of sky-high progesterone (worsening constipation, reflux, and fatigue), rising estrogen (swelling, breast tenderness), and elevated cortisol (anxiety, disrupted sleep) makes the final six to eight weeks physically and emotionally demanding in ways that feel distinct from the nausea-dominated first trimester. Your body is also producing increasing amounts of oxytocin receptors in the uterus and ramping up prostaglandin activity to prepare for labor, which can cause Braxton Hicks contractions and a general feeling of restlessness.
Mood Changes and Brain Chemistry
Progesterone doesn’t just affect your gut and energy. Your body converts some of it into a compound that acts as a natural sedative and anti-anxiety agent by calming certain receptors in the brain. Levels of this compound rise steadily throughout pregnancy, and your brain adapts by dialing down the sensitivity of those receptors. This is a normal adjustment, but it has consequences.
Women who produce lower amounts of this calming compound during the second trimester have a significantly higher risk of postpartum depression. One study found that for every additional unit of this compound measured in the second trimester, the risk of postpartum depression dropped by 63%. The theory is that when hormone levels crash after delivery, some women’s brains are slower to restore normal receptor function, leaving them vulnerable to severe mood disruption during the postpartum weeks.
Mood swings during pregnancy itself tend to cluster around two windows: the first trimester, when hormones are shifting rapidly and hCG is at its peak, and the third trimester, when cortisol and progesterone are both elevated. The second trimester is generally the most emotionally stable period.
The Postpartum Crash
The most dramatic hormonal event of the entire pregnancy actually happens after delivery. When the placenta detaches, estrogen and progesterone levels plummet. Both hormones drop massively in all women within the first few days after birth and continue to decrease through the first postpartum week. Most women remain in very low hormone ranges through at least eight weeks postpartum, though some show small fluctuations starting around days four and five.
This abrupt withdrawal after months of extremely high levels is what drives the “baby blues,” which affect up to 80% of new mothers in the first two weeks. For about 10 to 20% of women, the crash triggers postpartum depression, particularly in those whose brains had already downregulated their calming receptors during pregnancy and couldn’t recover quickly enough after the hormonal drop. The postpartum period is, in many ways, when the hormonal impact is most severe, even though it’s technically no longer pregnancy.