There are three main hormones involved in establishing and maintaining a pregnancy, one of which is new to your body. This new hormone is called human choriogonadotropin-beta (hCG-beta) and is made by the embryo as it burrows into the lining of your uterus. To be more precise, it is made by the placental cells of the embryo. These are the cells that do most of the burrowing and are in contact with your blood supply. These placental cells release hCG-beta into your bloodstream and this sends a signal which prevents the breakdown of the corpus luteum. The corpus luteum is a type of harmless cyst which is left over from ovulation and it secretes progesterone. If pregnancy doesn’t occur, it will eventually break down and this decrease in progesterone triggers menstruation. But if pregnancy does occur, the hCG-beta in your bloodstream tells the body to maintain the corpus luteum. Levels of hCG-beta increase rapidly until around the 12th week of pregnancy and after that they markedly decrease and then level off. The reason for this is that by the 12th week, the placenta is able to take over production of progesterone and consequently, there is no need for hCG-beta to stimulate the corpus luteum.
Progesterone is one of the key hormones for maintaining a pregnancy. Following implantation, levels of progesterone steadily rise, and unlike hCG-beta, they continue to rise until birth. It is this hormone which makes you feel so tired in the first trimester. In early pregnancy, progesterone:
- Increases blood flow to the womb by stimulating the growth of existing blood vessels
- Stimulates glands in the uterus lining to produce nutrients that nourish the early embryo
- Helps establish the placenta
In mid-to-late pregnancy, some of progesterone’s important functions include:
- Aiding the development of your baby
- Preventing your breast from secreting milk until after the birth
- Preventing your uterine muscles from contracting until the beginning of labor
- Strengthening the muscles of the pelvic wall in preparation for labor
Progesterone also helps to suppress the immune system during pregnancy so your immune system doesn’t attack the baby and slows down your digestive system. This is to enable you to extract the maximum amount of nutrients from your food, although it can lead to the unpleasant side effects of constipation and heartburn.
The third main hormone involved in maintaining a pregnancy is estrogen. Although progesterone is the dominant hormone during pregnancy, estrogen is still very important. As with progesterone, estrogen is made by the corpus luteum in the beginning of pregnancy and then later on the fetus produces a hormone called estriol. Estriol is then transferred to the placenta where it is made into several different types of estrogen. Similar to progesterone, levels of estrogen rise steadily until birth.
Some of the functions of estrogen are:
- Maintaining, controlling, and stimulating the production of other pregnancy hormones
- Stimulating the growth of the placenta and ensuring it functions correctly
- Enabling correct development of your baby’s organs, such as the kidneys, liver, and lungs
- Promoting the growth of breast tissue