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Hormone Handbook

During pregnancy, many of the changes you’re going through are visible—your growing breasts and belly are the most obvious. Others, like a powerful urge to “nest,” you can’t see but can certainly feel. A great number of these changes are due to hormones, powerful chemicals that affect your mind, your body and your pregnancy. Here’s a guide to some of the most important players.

Human chorionic gonadotropin (hCG). This hormone stimulates the corpus luteum (the part of the egg follicle left behind in the ovary after ovulation) to produce estrogen and progesterone in the first 10 weeks after conception, until the placental cells can do so themselves. For this reason, it is also the hormone we check in your urine or blood to determine conclusively whether you’re pregnant.

Progesterone. This hormone serves the very important role of quieting the uterine muscle so that the fertilized egg can have a safe landing while implanting. Progesterone levels increase throughout pregnancy to help maintain the inner layer of the uterus so it can provide support for the developing fetus. The hormone also creates a tranquilizing effect to protect against stress, one of the reasons pregnant women can handle so much heavy thinking and anxiety. In this way, it also promotes sleep. On the downside, progesterone has been linked to acne breakouts.

ON FIT PREGNANCY: Have a Better Pregnancy

Estrogen. Increased estrogen, which you may associate with changes (both positive and negative) in your mood and sex drive, also stimulates the growth of your uterus and improves blood flow between the uterus and placenta by enhancing the effect of nitric oxide, a gas that widens blood vessels. Not only does more estrogen mean better blood flow, it also prepares your breasts for milk production by enlarging the milk ducts. Estrogen secretion peaks right before birth and declines afterward.

ON FIT PREGNANCY: Nesting Instinct

Relaxin. Relaxin does just what you’d think. Most importantly, it relaxes the intrauterine ligaments. Why? Because if flexibility in the fetus’s living quarters isn’t increased, trying to grow a baby in the uterus would be like trying to squeeze a mountain lion into a file cabinet. This hormone also loosens things up a bit down below to ease the baby’s passage through the birth canal. It relaxes the arteries as well, so they can handle pregnancy’s increased blood volume without sending your blood pressure through the roof. The downside is that things you don’t want relaxed are sometimes affected. For instance, what happens when the muscle that prevents stomach acid from creeping back into your esophagus becomes floppy? You got it: heartburn.

ON FIT PREGNANCY: A Bump in the Road

Oxytocin. Oxytocin is the feel-good hormone that helps us bond with others. In the days and weeks immediately before delivery, many women 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. After delivery, when a woman holds her newborn, she develops what’s called “baby lust,” a chemical reaction that happens when a baby’s pheromones stimulate the production of additional oxytocin—thus augmenting the mother-baby bond.

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