Pregnancy

Pregnancy is an amazing process that affects almost every body system.

Throughout the pregnancy, estrogen and progesterone levels steadily rise, and it leads to a number of anatomic and physiologic changes that occur throughout the body.

Everything starts with ovulation, so let’s call that day 0.

On that day, in the ovary, an ovarian follicle – which is an egg or oocyte plus its surrounding tissues– matures and ovulation occurs which is when the egg gets ejected while the surrounding structure becomes the corpus luteum and quickly starts making estrogen and progesterone.

Normally, the egg gets fertilized by a sperm within 12-24 hours to form a zygote, so let’s say that fertilization happens a day later on day 1.

Almost right away, cells start to divide over and over, until there’s a ball of cells called the blastocyst on day 4.

The blastocyst typically floats around inside the uterus for another day before it finds a specific spot to implant on day 5.

Around this time, the corpus luteum makes a lot more progesterone relative to estrogen, and the low estrogen to progesterone ratio is necessary for implantation.

At this early stage, there are two parts to the blastocyst - an inner set of cells that go on to become the fetus, and an outer set of cells called the trophoblast that burrow into the endometrium on day 6 and eventually develop into the fetal part of the placenta.

That trophoblast cells start to produce a hormone called human chorionic gonadotropin or HCG around day 8, and this is important for two reasons.

One - it’s the hormone that lets the corpus luteum know that there has been a successful implantation into the endometrium, and that it should continue to make estrogen and progesterone.

And it’s the continued presence of estrogen and progesterone that suppresses other ovarian follicles from maturing.

Two - HCG is the hormone that most pregnancy tests are able to detect, causing the little sign to form which can happen as early as day 9.

Without HCG levels shooting up on day 8, the corpus luteum would start to shrivel up by day 10, and estrogen and progesterone levels would fall.

This would cause the lining of the endometrium to slough off or fall away from the endometrial wall resulting in a period or menses.