The hormone most often called “the pregnancy hormone” is human chorionic gonadotropin, or hCG. It’s the hormone that home pregnancy tests detect in your urine, and it’s produced almost exclusively during pregnancy. But hCG is just one player in a larger hormonal system. Progesterone, estrogen, and several other hormones all rise dramatically during pregnancy, each with a distinct job in keeping you and the fetus healthy.
hCG: The Hormone Pregnancy Tests Detect
hCG is what makes a pregnancy test turn positive. Your body starts producing it shortly after a fertilized egg implants in the uterine wall, and low levels can appear in your blood as early as 6 to 10 days after ovulation. Most home pregnancy tests are designed to detect hCG at concentrations of 25 mIU/mL or higher, though some early-detection tests can pick up levels as low as 10 mIU/mL.
What makes hCG useful as a pregnancy marker is how fast it rises. At around 3 weeks after your last menstrual period, levels sit between 5 and 50 mIU/mL. By weeks 7 to 8, they can climb to anywhere from 7,650 to 229,000 mIU/mL. Levels peak somewhere between weeks 9 and 12, then gradually decline for the rest of the pregnancy. This is why pregnancy tests become more reliable with each passing day after a missed period: there’s simply more hCG to detect.
Beyond triggering a positive test, hCG has a critical biological function. In early pregnancy, it signals the corpus luteum (a temporary structure in the ovary) to keep producing progesterone. Without that signal, progesterone would drop, the uterine lining would shed, and the pregnancy would end. Once the placenta is developed enough to produce its own progesterone, around the end of the first trimester, hCG’s job is largely done, which is why levels taper off.
Progesterone: Maintaining the Uterine Lining
If hCG is the signal, progesterone is the workhorse of early pregnancy. It stimulates the thickening of the uterine lining so a fertilized egg can implant and grow. Throughout pregnancy, progesterone keeps that lining stable, prevents the uterus from contracting too early, and helps suppress the immune response that might otherwise reject the embryo as foreign tissue.
Progesterone levels rise steadily through all three trimesters. In the first trimester, the corpus luteum produces most of it. By the second trimester, the placenta takes over entirely. The dramatic drop in progesterone after delivery is one of the major hormonal shifts that defines the postpartum period.
Estrogen: Supporting Growth and Blood Flow
Estrogen levels during pregnancy climb to concentrations far higher than at any other point in your life. Produced first by the ovaries and then by the placenta, estrogen helps maintain the pregnancy and supports the development of the fetus’s organs and body systems. It also increases blood flow to the uterus, which is essential for delivering oxygen and nutrients as the fetus grows.
Many of the visible changes during pregnancy, from skin changes to breast growth, are driven at least partly by estrogen. Like progesterone, estrogen drops sharply after delivery, contributing to the hormonal upheaval of the postpartum weeks.
Human Placental Lactogen: Redirecting Your Metabolism
Once the placenta is established, it produces a less well-known hormone called human placental lactogen (hPL). This hormone adjusts your metabolism so that more nutrients stay available in your bloodstream for the fetus. One of its key effects is making your body less sensitive to insulin, the hormone that normally moves sugar from your blood into your cells. By dialing down that insulin response, hPL keeps more glucose circulating for the developing baby to use.
This metabolic shift is a normal part of pregnancy, but in some people it tips too far, contributing to gestational diabetes. That’s one reason glucose screening is a routine part of prenatal care.
Relaxin: Loosening Joints for Delivery
Relaxin does exactly what its name suggests. It relaxes the ligaments and joints of the spine and pelvis, gradually increasing flexibility so the body can accommodate a growing uterus and eventually allow the baby to pass through the birth canal. Relaxin levels rise early in pregnancy and remain elevated throughout.
The downside of all that loosening is that it can cause lower back pain, pelvic discomfort, and a feeling of instability in your joints. These are among the most common physical complaints during pregnancy, and relaxin is a major reason why.
Oxytocin and Prolactin: Labor and Breastfeeding
Two hormones take center stage at the very end of pregnancy and into the postpartum period. Oxytocin triggers the strong, rhythmic contractions of the uterus during labor. Synthetic oxytocin is often used to induce or strengthen labor when contractions aren’t progressing on their own.
Prolactin controls milk production. Your body begins producing prolactin early in pregnancy, but the high levels of estrogen and progesterone keep it suppressed. Once you deliver the placenta and those pregnancy hormones drop, prolactin takes over and milk production begins. After that, the cycle is maintained by breastfeeding itself: when your baby suckles, it triggers nerve signals that tell your body to release more prolactin (to make milk) and oxytocin (to push milk through the ducts).
What Happens to These Hormones After Delivery
The postpartum hormonal shift is one of the steepest your body will ever experience. Estrogen and progesterone, which climbed for nine months, plummet within hours of delivering the placenta. hCG drops to undetectable levels within a few weeks. This sudden withdrawal is a major factor in postpartum mood changes, including the “baby blues” that affect most new parents in the first two weeks.
Hormone levels begin stabilizing between 2 and 3 months postpartum and typically return to pre-pregnancy baselines by around 6 months. If you’re breastfeeding, prolactin stays elevated longer, which can delay the return of your menstrual cycle and keep estrogen levels lower than they were before pregnancy.