Hormones during the First Trimester of Pregnancy

Reviewed by Beth Iovinelli in June 2010

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Now that you're pregnant your hormones seem to be going crazy! This overview will help explain the biological reasons behind your surging hormones.

Over the next nine months you're in for a wild time! If pregnancy were an amusement park ride it might start off as the merry-go-round. By the second trimester you're on the tilt-a-whirl, and by the third trimester you're riding the big coaster! Fortunately, since pregnancy takes nine months, you'll have some time to get used to all of the ups and downs. What exactly are these hormones doing inside your body?

Estrogen, progesterone, hCG, hPL, and prolactin are the major pregnancy hormones. Thyroid and adrenal hormones from both you and your baby also play roles. Let's take a look at the five big players.

Estrogen

Estrogen is a female hormone produced in the ovaries. During your menstrual cycle, estrogen and progesterone levels cycle up and down, falling to their lowest levels while you are menstruating. Estrogen and progesterone stimulate the endometrium to support a fertilized egg if pregnancy occurs. During the first trimester, estrogen production is mostly made by the placenta, and this continues until near the end of the pregnancy when the amount circulating in your body is a thousand times the amount you have when not pregnant.

By improving the blood supply, estrogen makes everything grow. Breasts enlarge, moles and skin tags get bigger, spider veins become more prominent, and blood vessel beds become swollen and engorged.

Progesterone

Unlike estrogen, which is manufactured by the placenta, progesterone comes from the cholesterol connection in your own blood. Progesterone is a potent anti-inflammatory, which may help to blunt your immune system so you won't reject your baby. It is also a smooth muscle relaxant, which can slow gastric emptying, giving you heartburn and acid reflux, and decreasing the tone of your bowels, leading to constipation. More bad news: it also increases the oiliness of your skin, so if you're the fairest in all the land, you'll be shocked to see that first zit in five years rear its ugly little head. Additionally, progesterone's relaxant properties may play a role in keeping your uterus from contracting until term.

Together, estrogen and progesterone help control the onset of breast milk, keeping the floodgates closed by inhibiting the breasts' response to the hormone prolactin. Estrogen and progesterone levels plunge at the time of delivery, allowing prolactin to stimulate the breasts' milk flow.