A Cheat Sheet to Pregnancy Hormones

Pregnancy hormones are an amazing and—at times—mysterious thing. These powerful chemicals don't just grow a human being, they affect your mind and body. They also play a big a role in things like morning sickness and heartburn. In short, your changing hormones will impact you (and your baby) in more ways than one. But knowledge is power, so read on as Michele Hakakha, M.D., an obstetrician and co-author of Expecting 411, guides you through the ways in which six key hormones change while you're expecting.

HCG

During pregnancy, hCG levels rise eight days after ovulation, peak at 60 to 90 days, and then lower slightly, leveling off for the remainder of the pregnancy. Typically, during the first 10 weeks of your pregnancy, hCG levels double every two days.

HCG circulates through the body and is eliminated in the urine (which is what over-the-counter pregnancy tests are looking for—a high concentration of beta hCG in the urine that indicates you are, indeed, pregnant).

What it does

Human chorionic gonadotropin (hCG) is the key hormone during pregnancy. It's produced by what ultimately becomes the placenta. The basic job of hCG is to tell the body that there's life growing in the womb and that the body needs to build a nest for it. It also tells the ovaries to shut off the production line of maturing an egg every month.

The down side

Symptoms of rising hCG levels can include fatigue, nausea/vomiting (aka morning sickness), dizziness or light-headedness, breast tenderness, and feeling emotionally sensitive. No one is entirely sure what causes morning sickness, but many doctors believe it is most likely connected to your rising hCG levels as pregnant people with higher levels of hCG often experience more nausea and vomiting.

Progesterone

In the first trimester, levels of progesterone rise exponentially, and then they plateau. Progesterone does some very important jobs along the way: It keeps the uterus muscle relaxed and helps your body's immune system tolerate foreign DNA (that is, the fetus).

What it does

Progesterone is made early in pregnancy by a cyst on the ovary called the corpus luteum. The corpus luteum continues to produce progesterone until about 10 weeks, when its production is taken over by the placenta.

The down side

Progesterone relaxes all smooth muscle (most important, the muscle wall of the uterus or "womb") in the body. It also leads to relaxation of the blood vessels throughout the body, prompting lower than normal blood pressure and occasionally dizziness, as well as gastrointestinal symptoms like heartburn, reflux, belching, nausea, vomiting, gas, and constipation. Progesterone can also increase hair growth. You may notice unwanted hair on your breasts and lower abdomen, for example.

Estrogen

Once you've reached the end of the first trimester, your body has higher levels of circulating estrogen, and then the levels plateau.

What it does

Like progesterone, estrogen is secreted by the corpus luteum until the placenta takes over. This pregnancy hormone plays a key role in the development of the fetus, and it triggers the growth of several organs and other bodily systems.

The role of estrogen is super-important: It helps to stimulate hormone production in the fetus's adrenal gland, stimulates the growth of the adrenal gland, and enhances the uterus, enabling it to respond to oxytocin (another pregnancy hormone). It also prepares your breasts for milk production by enlarging the milk ducts.

The down side

Elevated estrogen levels, however, may prompt spider veins, nausea, increased appetite, and skin changes. That said, some are lucky enough to experience the upside of a pregnancy 'glow,' which is largely attributed to estrogen levels.

Relaxin

While expecting, people have 10 times the normal amount of relaxin in their bodies.

What it does

Relaxin is believed to be responsible for loosening the ligaments that hold the pelvic bones together and for relaxing the uterine muscle. This prepares your body for baby's passage through the birth canal. The pregnancy hormone also relaxes the arteries, so they can handle pregnancy's increased blood volume without sending your blood pressure through the roof.

The down side

You may feel that your ligaments are 'looser,' including your shoulders, knees, hips, and ankles, which can result in aches, pain, inflammation, and even clumsy tendencies.

Oxytocin

Many believe oxytocin is the hormone that triggers labor. (Pitocin, the drug usually given to induce labor, is the synthetic form of oxytocin.) In truth, oxytocin levels don't rise as labor begins; the uterus simply becomes very sensitive and responsive to oxytocin as you progress toward the end of pregnancy.

What it does

Oxytocin is also a hormone that stretches, though it stretches the cervix (not the pelvis) and stimulates the nipples to produce milk.

In the days and weeks immediately before delivery, some pregnant people experience mild euphoria and strong nesting behavior (inexplicably washing walls, baking, and so on), and this may be linked to oxytocin as well as to other hormones and steroids. During delivery, huge bursts of oxytocin run through the brain.

Prolactin

Prolactin levels increase 10 to 20 times during pregnancy.

What it does

This milk-producing hormone has a tranquilizing effect. Prolactin prepares breast tissues for lactation and the release of milk.