Hormones during pregnancy

Pregnancy involves some of the largest and fastest hormonal changes the body ever undergoes. New hormones appear, familiar ones rise to levels not seen at any other time, and together they support the pregnancy, prepare the body for birth, and ready it for feeding a newborn.

A rapidly changing hormonal environment

Shortly after a fertilized egg implants, the developing placenta begins producing human chorionic gonadotropin (hCG), the hormone home pregnancy tests detect. hCG signals the ovaries to keep producing progesterone in early pregnancy. As the placenta matures, it becomes a powerful hormone-producing organ in its own right, taking over much of this work and adding hormones of its own.

The result is a steadily shifting internal environment across the three trimesters. Many hormones climb throughout pregnancy and then fall sharply after delivery — a pattern that helps explain both the experiences of pregnancy and the adjustments of the weeks that follow.

The characteristic hormonal changes of this stage

What sets pregnancy apart hormonally is the arrival of a temporary new organ — the placenta — that acts as an endocrine gland for the duration of the pregnancy. In the first weeks, hCG dominates and props up the ovary's production of progesterone. As the placenta grows, it takes over production of progesterone and estrogen and sustains them at high levels until birth. Because the placenta is doing this work, the usual brain-driven menstrual cycle is effectively paused: ovulation and periods do not occur during pregnancy.

The changes unfold in a recognizable arc across trimesters. The first trimester is shaped by the rapid early climb in hCG and the experiences associated with it. The second trimester often feels steadier as those early surges level off and the placenta settles into its role. The third trimester brings the hormonal and mechanical preparations for labor and feeding, including changes that gradually ready the cervix and uterus and that prime the breasts for milk production.

The main hormones involved

Progesterone rises substantially. It helps maintain the uterine lining, relaxes smooth muscle (which can slow digestion and relax blood vessels), and supports the pregnancy. Estrogen, mainly estriol during pregnancy, also rises and supports growth of the uterus and breasts and the developing blood supply.

Prolactin increases to prepare the breasts for milk production. Relaxin helps loosen ligaments and joints in preparation for birth. Thyroid hormones shift, and the thyroid gland normally works harder during pregnancy. Cortisol and insulin-related changes also occur, as the placenta produces hormones that alter how the body handles blood sugar.

Oxytocin becomes increasingly important toward the end of pregnancy and during labor, where it contributes to the contractions of birth, and afterward to the milk let-down reflex. The interplay of all of these hormones — rather than any single one — is what allows the body to support a growing pregnancy, adapt the heart, kidneys, and blood volume to extra demand, and prepare for the transition to feeding a newborn.

What is typically experienced

Early pregnancy is commonly marked by fatigue, nausea (which many associate with rising hCG), breast tenderness, and frequent urination. As pregnancy continues, the relaxing effect of progesterone on smooth muscle can contribute to heartburn, constipation, and nasal congestion. Rising blood volume and changing circulation can cause swelling and lightheadedness.

Skin changes such as darkening of certain areas, hair changes, mood shifts, vivid dreams, and changes in appetite are all common and largely hormonally driven. Toward the end of pregnancy, hormonal and mechanical signals help prepare the cervix and uterus for labor. The wide range of experiences is normal — some people feel many of these changes intensely and others very little.

Common experiences and the hormones behind them

Many familiar features of pregnancy map onto specific hormonal shifts. The loosening of ligaments that can make the pelvis and lower back feel different reflects the action of relaxin. Slower digestion, heartburn, and constipation often follow progesterone's relaxing effect on smooth muscle. Breast changes track the rise in estrogen, progesterone, and prolactin as the body prepares for feeding. Even the increased need to urinate has both hormonal and mechanical contributions. Understanding these links can make the experiences feel less mysterious, though it does not replace guidance tailored to an individual pregnancy.

Hormone-related conditions in pregnancy

Because pregnancy reshapes so many hormonal systems, some conditions are specific to or more common in pregnancy. The placenta's effect on blood sugar can lead to gestational diabetes in some people, which is why glucose screening is a routine part of prenatal care. Thyroid function can shift in either direction and is sometimes checked. These situations are generally manageable, especially when identified through routine prenatal care.

Related tests a clinician might use

Prenatal care draws on a standard set of checks, several of which involve hormones or the systems they regulate. hCG may be measured to confirm and, in some situations, monitor an early pregnancy. A glucose screening test is a routine way to look for gestational diabetes, and where blood sugar needs longer-term tracking a clinician may also consider a hemoglobin A1c test. Thyroid tests such as TSH are sometimes used, particularly when there is a relevant history or symptoms.

Beyond hormone-specific tests, routine prenatal visits include blood pressure checks, urine tests, and other monitoring that can flag conditions linked to the hormonal and circulatory changes of pregnancy. Which tests are appropriate, and when, is individualized — a clinician or midwife decides based on the pregnancy, history, and any symptoms.

When to consider talking to a clinician

Routine prenatal care is the main way hormone-related issues are monitored, so keeping scheduled visits matters. Beyond that, it is worth contacting a clinician promptly about severe or persistent vomiting that prevents keeping fluids down, severe headaches, marked swelling that comes on suddenly, vision changes, very high thirst and urination, or any symptom that feels alarming. Persistent low mood, anxiety, or distress during pregnancy also deserve attention and support.

Related reading

See the related postpartum hormone changes guide for what happens after birth, browse all life-stage guides, or read about individual hormones and the blood tests used to measure them.