The Hormonal Changes During Pregnancy No One Tells You About

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You're pregnant! Congratulations! Now what happens?

Whether your journey to pregnancy has been a roller-coaster ride or a pleasant surprise, all women go through similar hormonal changes during pregnancy. The female body is absolutely incredible, resilient, and intuitive when it comes to building another human being. It knows exactly what to do, even if you're unsure of what might be happening in your body. In this post I will guide you through all of the hormonal changes during pregnancy so you know exactly what's going on. After all, knowing what is happening in your body can make you feel more empowered and in control during your pregnancy.

It all starts with an egg breaking free from the ovary during ovulation. If that egg is fertilized with sperm, the fertilized egg begins differentiating into a group of cells called a trophoblast. This occurs about six days after ovulation. It is this trophoblast that produces the pregnancy detection hormone known as human chorionic gonadotropin (hCG). On average, hCG is detectable in your urine 10 days after ovulation/fertilization.

1. hCG

When your egg ruptured from the ovary, it left behind its outer coat called the corpus luteum. It is the job of hCG to maintain the corpus luteum so that the corpus luteum can make progesterone and estrogen to support embryonic growth. If your egg is not fertilized and you do not produce hCG, the corpus luteum slowly disintegrates and you will get your period. hCG is present throughout your whole pregnancy but reaches peak levels around week nine to ten; at this point the placenta is able to take over hormone production.

2. Estrogen

Estriol (E3) is the predominant form of estrogen during pregnancy. Cholesterol from your circulation enters the placenta and is required to make all steroid hormones. The production of estrogen depends on the mother and the fetus. You work together with your baby to make estrogen! Here's how it happens:

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  • Cholesterol enters the placenta and turns into pregnenolone
  • Pregnenolone is converted into DHEA in the adrenal gland and liver of the fetus
  • DHEA crosses back into the mother's circulation
  • Mommy uses the enzyme aromatase to convert DHEA into estriol

Phew!! Now that's teamwork! What does estrogen do?

  • Develops breast tissue
  • Supports the lining of the uterus
  • Increases blood flow to the uterus—and everywhere else, which can cause rashes and venous stasis
  • Stimulates fetal organ development
  • Relaxes smooth muscle (organs)—causing reflux and constipation
  • Closer to term, it stimulates prostaglandins to promote uterine contractions and helps ripen the cervix with the hormone DHEA

3. Progesterone

Compared to estrogen, progesterone production is easy! It is made entirely by the placenta. Mommy doesn't have to do anything. Just like estrogen, progesterone is made from cholesterol. It is first converted to pregnenolone, and then it becomes progesterone.

What does progesterone do?

  • Allows for implantation of embryo—if you've gone through IVF you may take a progesterone supplement during your third trimester to ensure embryo stickiness!
  • Suppresses the maternal immune response—this prevents rejection of the fetus and can stabilize immune conditions like multiple sclerosis
  • Maintains the placenta
  • Supports the lining of the uterus
  • Prevents uterine contractions
  • Relaxes smooth muscle—causing reflux and constipation
  • Loosens pelvis ligaments to prepare for labor

4. Thyroid Hormone

There are two thyroid hormones to take note of during pregnancy. T4 is an inactive, precursor hormone. T3 is your active thyroid hormone, which is derived from T4. Elevated estrogen levels promote elevated T3 and T4 to help with metabolism and regulation of steroid hormones.

5. hPL

Human placental lactogen is secreted by the placenta in order to help mom break down fats and carbohydrates for the baby. Unfortunately, this hormone opposes insulin so it can lead to insulin resistance in mom; this is why you may be screened for gestational diabetes at the end of the second trimester.

6. Cortisol

Cortisol is your "stress" hormone. As you near 40 weeks of pregnancy, cortisol production from the baby's adrenal glands increases. This increase makes the uterus more sensitive to contractions; it also increases estrogen in the circulation and decreases progesterone to allow for even more contractions! You may be familiar with Braxton Hicks contractions, which are false contractions that occur from pressure on the uterus, not from elevated estrogen.

7. Oxytocin

Get ready to push! Oxytocin is made by mommy and is responsible for dilating the cervix and stimulating strong uterine contractions for delivering baby. After delivery, oxytocin is stimulated by the baby suckling on the nipple and promotes milk let down.

8. Prolactin

Prolactin is the hormone that stimulates your breast tissue to make milk. Estrogen promotes prolactin secretion from mom's pituitary gland to prepare for lactation, but you will not typically begin lactating until after birth because the high amounts of estrogen and progesterone inhibit prolactin from acting on the breast tissue. After delivery when the hormone levels decrease, prolactin can then promote lactation. When the baby sucks on the nipple, prolactin and oxytocin are secreted to produce milk. Prolactin can be a "natural birth control" if you produce enough of it and breastfeed frequently; this is because prolactin inhibits ovulation.

Hormones begin to normalize immediately after the delivery of the placenta, and are back to pre-pregnancy levels by day five post-labour. That's really quick! It is this spontaneous change in hormones that can lead to low mood in the postpartum period. Being more aware of the influence of these hormones on your body will help you understand what you're feeling and hopefully help you cope with the symptoms.


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