How Pregnancy Hormones Affect Your Body in Each Trimester

The first trimester can be a doozy. Your body is going into overdrive to grow that tiny embryo. Your blood volume increases, your immune system changes to protect the fetus, and your bloodstream is coursing with a whole whack of hormones.

When you first become pregnant, progesterone and estrogen, hormones that are a part of your normal menstrual cycle, rise dramatically, and a new hormone, one that’s special to pregnancy, called human chorionic gonadotropin, begins to be produced.

Progesterone is important for getting pregnant in the first place because it prepares your uterus lining for the egg to implant and it acts as a muscle relaxant, preventing your uterus from contracting until the onset of labour. But those same muscle-relaxing properties can also cause constipation, as it slows down your digestive tract, says naturopathic doctor Kinga Babicki-Farrugia.

Progesterone is also associated with that classic irritability in the premenstrual period—and because your progesterone levels stay elevated during your pregnancy, mood swings can be a side effect, explains Ilana Halperin, a staff physician in endocrinology at Sunnybrook Health Sciences Centre and assistant professor at the University of Toronto.

Schwartz’s mood swings are actually what triggered her to take a pregnancy test in the first place. She would be angry one minute and sad the next.

Meanwhile, estrogen helps to regulate that progesterone while also maintaining the endometrial lining that is vital for the baby’s development. Estrogen is believed to promote an increase in blood flow, which is important for nourishing the baby, but that extra blood flow has the side-effect of making your breasts achy and tender, says Babicki-Farrugia.

The increase in blood flow can also lead to that stuffy nose many women experience in pregnancy, by causing the mucous membranes in your nasal passages to swell. And that same blood increase can cause you to head to the bathroom more often.

As your kidneys process the extra fluids and your uterus puts pressure on your bladder, you might feel the urgent and frequent need to pee. Fortunately, there is at least one benefit that may be due to this increase—a boost in blood flow may be part of the reason for your skin’s healthy pregnancy glow.

This is a good time to break out extra-absorbant underwear, you know—the kind designed for periods. You know what they say, leaks happen. You'll live in these in the first few weeks after baby is born.

Human chorionic gonadotropin, or hCG, is known as the pregnancy hormone because it’s generally only produced during pregnancy. Home pregnancy tests give you that positive stripe when they detect this hormone in your urine. When you’re pregnant, your levels of hCG will rise rapidly, doubling every few days before reaching their peak in the first eight to 11 weeks.

This hormone is important because high levels of it indicate that the placenta is being created, says Doug Wilson, the department head of obstetrics and gynecology at Calgary’s Foothills Medical Centre and a professor at the University of Calgary. But, it may also make you feel nauseous.

While there’s no conclusive science on exactly what causes morning sickness, hCG is often thought to be one of the culprits. “I always tell my patients that the sicker you feel, the better it means that your implantation and placenta is working,” says Wilson. If you are pregnant with twins or multiples, you may experience more nausea because there’s more hCG being created by your body.

You may also have a heightened sense of smell, which could be related to hCG levels and increase your nausea. Since hCG peaks around weeks eight to 11, some women (unfortunately not all) start to feel some relief once they begin the second trimester.

You might not think your thyroid has anything to do with pregnancy, but when you’re expecting, these hormones also go through a rapid change. Your thyroid hormones help regulate the metabolism of every cell in your body. When you become pregnant, those thyroid hormones need to increase in order to support the baby’s neurodevelopment and bone development, says Halperin.

Thyroid-stimulating hormone, or TSH, is produced in the pituitary gland in the brain and it helps regulate your other important thyroid hormones. The increase in hCG and estrogen can stimulate the thyroid even more, especially in someone expecting twins or multiples.

“It’s a protective mechanism,” says Halperin, making sure your baby is getting enough of the thyroid hormones it needs. Your doctor will measure your thyroid levels through blood word in the first trimester, and may check again every four to six weeks if there is a known thyroid problem or an abnormality in the initial test. If anything is out of the ordinary, they may refer you to an endocrinologist for an assessment

The second trimester is commonly known as the “best trimester” because at around week 13, many women start to feel human again. Nausea often starts to dissipate, but you’ll start to notice other changes.

For one thing, your muscles and joints may feel strange, or uncomfortable, especially around your pelvis. This is thanks to relaxin, a hormone that helps relax the smooth muscles in the pelvis, such as the cervix and uterus, and promote the growth of the placenta. While this might make prenatal yoga a bit easier, unfortunately, it can also cause women to experience some aches and pains in their ligaments, or even experience injuries more easily.

Schwartzexperienced serious back pain and soreness in her right hip and pelvis during her second trimester, likely thanks to this hormone. “I could barely walk,” she says. “I would drive home from work and I couldn’t get out of the driver's seat for half an hour.” The pain didn’t dissipate until she gave birth.

In the second trimester, estrogen and progesterone continue to increase to help grow the baby. But these hormones also stimulate the melanocyte-stimulating hormone. It triggers your skin’s melanocyte cells to produce melanin, which gives your skin colour. This is why some women may begin to notice the “mask of pregnancy,” otherwise known as melasma, which causes brown or grey patches around the face.

You may also notice a dark line appear that goes up your belly, called the linea nigra, and your nipples darkening, along with more pronounced moles and freckles. Luckily, it usually resolves post-partum. Your hair might also begin growing more rapidly at this stage thanks to these same estrogen surges.

Cortisol also increases during pregnancy, but despite what you hear about this stress hormone, it’s not all bad—it’s important for the developing fetus because it can help regulate your metabolism and control blood sugar levels. 

High levels of cortisol may be also associated with some of those unfortunate symptoms like stretch marks, blood pressure issues and added redness in the face and cheeks, says Babicki-Farrugia.

Human placental lactogen (HPL), a hormone secreted from the placenta, is thought to help the baby grow. It’s also one of the main hormones connected to insulin resistance during pregnancy, or gestational diabetes, which sometimes develops in the second trimester and can lead to overgrowth of the baby.

Your body’s changing reaction to insulin makes a lot of sense evolutionarily, says Halperin. “It’s a time of rapid growth for the fetus. If you had a woman who wasn’t getting enough to eat, by making them insulin resistant, more blood sugar, which is the mainstay for growth for the fetus, would pass over to the placenta,” she explains.

Your baby is starting to pack on the pounds this trimester and your body is increasing some of the hormones it will need postpartum. Estrogen and progesterone peak around 32 weeks and your estrogen levels are the highest they will ever be during this trimester—six times higher than before pregnancy, says Babicki-Farrugia. This is definitely the time to start taking your newborn checklist seriously.

In this trimester, you might notice lots of swelling around your ankles and feet. While this can be related to the lymphatic system, estrogen may also play a role because it’s indirectly involved in synthesizing a hormone related to salt and water retention, says Babicki-Farrugia.

Late in pregnancy, women can also experience acid reflux or heartburn, because progesterone has relaxed the sphincter at the base of the esophagus, allowing food and stomach acid to travel back up. Meanwhile, relaxin helps to loosen those pelvis muscles towards the end of pregnancy to prepare for delivery.

Schwartz had bad acid reflux in her third trimester. “I lived on antacids,” she said. She also experienced such swollen feet that she couldn’t wear her regular shoes.

Prolactin, a hormone that stimulates the development of your breast tissue to prepare for lactation, ramps up in the third trimester. Prolactin is 10 times higher at the end of pregnancy than it is at the beginning.  

Though your body doesn’t actually produce milk at the breast until progesterone and estrogen drop after birth, it does start to get ready with colostrum, the first milk you produce for the baby, while you’re still pregnant. Your breasts may even start to leak a little bit before the baby is born, says Babicki-Farrugia.

Doctors don’t know exactly what triggers labour, but it is thought to be a complex cascade of events including a rise in the hormone oxytocin and a drop in progesterone. In fact, if you need to be induced, your doctor may give you synthetic oxytocin.

Oxytocin also produces the contractions your body needs to eliminate the placenta post-birth. Oxytocin and estrogen help release prostaglandins, which may soften the cervix to prepare for birth, and relaxin ramps up to loosen the ligaments and further soften and open the cervix.

It's a good idea to stock up on washable, reusable organic nursing pads at this point. These inexpensive little wonders will be your breast friend in the first several weeks after baby arrives, too.

After you give birth, your endorphins, those feel-good hormones that help you manage pain, are running high for 24 hours. “Most women feel superhuman—you’ve just delivered this incredible little person,” says Babicki-Farrugia. But by day three and four, your hormones take a serious nosedive. After your body releases the placenta, all the hormones it was producing such as estrogen, progesterone, relaxin, hCG and HPL, go with it, explains Wilson.

Estrogen and progesterone are the lowest they will ever be until you hit menopause. Many women start feeling those post-baby blues. You’re also likely sleep-deprived. Sleep deprivation is very clearly linked to cortisol, the stress hormone, so that also impacts the way you feel, says Halperin. But hopefully, you’ve got some oxytocin coursing through your system from quality baby time to help with some of the postpartum sadness.

For approximately eight percent of women in Canada, those baby blues will turn into postpartum depression. Schwartz dealt with postpartum depression after her son was born and struggled with it for almost two years. After a few months, when her symptoms hadn’t improved, her doctor prescribed antidepressants to help her body get back on track.

But postpartum hormone changes aren’t all bad. When the baby starts suckling, prolactin increases and it stimulates the production of milk, an important feedback loop when you are establishing your milk supply, says Wilson. Your prolactin levels also go up at night, so as exhausting as those nighttime feeds are, they can actually help establish your milk supply.

Breastfeeding and skin-to-skin contact also trigger the release of oxytocin, which is sometimes known as the “love hormone” because it increases in response to physical touch and is thought to help promote bonding. Oxytocin helps with milk letdown when breastfeeding, but, since the hormone can stimulate uterine contractions, some women may also experience painful cramps during breastfeeding for a few weeks after birth.

Eventually, for most women, everything will go back to normal, or your new normal, when your menstrual cycle returns, says Babicki-Farrugia . That could take up to a year or even longer and your hormones won’t level out completely until you’ve weaned.

Within six weeks of taking antidepressants, Schwartz says a light went off and she began feeling like herself again. While pregnancy was definitely a challenge, her health is finally back on track. She’s off her medication and loves spending quality time with her young son, who is now two.

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