The pregnancy hormone is called human chorionic gonadotropin, almost always referred to as hCG. It’s the hormone that pregnancy tests detect, and it plays a critical role in keeping a pregnancy viable during the earliest weeks. But hCG isn’t the only hormone involved in pregnancy. Several others work alongside it to support fetal development, prepare your body for delivery, and maintain the pregnancy from implantation through birth.
What hCG Does in Early Pregnancy
hCG’s primary job is to keep the corpus luteum alive. The corpus luteum is a small, temporary structure in the ovary that forms after ovulation, and it produces progesterone, the hormone that maintains the uterine lining so an embryo can implant and grow. Without hCG signaling the corpus luteum to keep working, progesterone levels would drop and the pregnancy would end.
This role is essential for roughly the first six weeks of pregnancy. After that point, the placenta takes over progesterone production on its own, and the corpus luteum is no longer needed. hCG levels continue to rise beyond that window, but the hormone’s most critical work happens in those early weeks when the placenta is still developing.
When hCG Appears and How Fast It Rises
hCG is produced by the cells that eventually form the placenta, starting almost immediately after the embryo implants in the uterine wall. It can be detected in blood as early as 3 to 4 days after implantation. In a healthy early pregnancy, hCG levels rise rapidly, increasing by at least 35% every two days. For very early pregnancies with low starting levels (under 1,500 mIU/mL), the expected minimum increase over 48 hours is around 49%.
Levels peak around weeks 10 to 12 of pregnancy, then gradually decline and stabilize for the remainder. This peak and drop is completely normal and explains why many first-trimester symptoms like nausea tend to ease as you move into the second trimester.
How Pregnancy Tests Detect hCG
Every home pregnancy test works by detecting hCG in urine. The difference between brands comes down to sensitivity, meaning how little hCG needs to be present for the test to register a positive result. These thresholds vary dramatically.
First Response Early Result has the lowest detection threshold at 6.3 mIU/mL, which is sensitive enough to catch over 95% of pregnancies by the day of a missed period. Clearblue Easy Earliest Results detects hCG at 25 mIU/mL, picking up about 80% of pregnancies at that same point. Most other brands require 100 mIU/mL or more, meaning they detect only about 16% of pregnancies on the day of a missed period. If you test early and get a negative result, the brand you used may simply not be sensitive enough yet.
Blood tests ordered by a doctor are more precise and can detect hCG earlier and at lower concentrations than any home test. They also measure the exact amount of hCG, which is useful for tracking whether levels are rising at a healthy rate.
What Abnormal hCG Levels Can Mean
Doctors sometimes monitor hCG levels with repeated blood draws, particularly if there are concerns about the pregnancy’s viability. Levels that rise more slowly than expected, plateau, or fall can indicate several possibilities: a miscarriage, an ectopic pregnancy (where the embryo implants outside the uterus), or a blighted ovum (where a gestational sac forms but no embryo develops). Sometimes low levels simply mean the pregnancy isn’t as far along as estimated based on the last menstrual period.
Higher-than-expected hCG levels can also carry meaning. The most common explanation is a twin or triplet pregnancy, since more placental tissue produces more hCG. Less commonly, unusually high levels can signal a molar pregnancy, a rare condition where abnormal tissue grows in the uterus instead of a healthy embryo. A single hCG reading on its own is rarely diagnostic. The trend over multiple measurements matters far more than any individual number.
Other Hormones That Support Pregnancy
While hCG gets the most attention because of its role in pregnancy testing, it’s part of a larger hormonal team. Progesterone, initially sustained by hCG’s effect on the corpus luteum, is arguably the most important pregnancy hormone overall. It maintains the uterine lining, prevents contractions, and supports the growing placenta. Without adequate progesterone, the uterus cannot sustain a pregnancy.
Relaxin is another hormone that rises during pregnancy. It loosens muscles, joints, and ligaments throughout your body to accommodate a growing uterus and increased blood volume. Early in pregnancy, relaxin helps prepare the uterine lining for implantation and relaxes blood vessels to handle the roughly 50% increase in blood volume that pregnancy demands. Later, it softens the cervix and loosens the pelvis to prepare for delivery. Relaxin is also responsible for the joint looseness and occasional instability many pregnant people notice, particularly in the hips and lower back.
Estrogen rises steadily throughout pregnancy as well, supporting blood flow to the uterus, helping fetal organs develop, and preparing breast tissue for milk production. Together, these hormones create the physiological environment that allows a pregnancy to progress from a single implanted embryo to a full-term baby.