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.
A pregnancy lasts 40 weeks, roughly 9 months, but that is from the last menstrual period, which is usually about 2 weeks before “day 0” of ovulation.
So if you’re counting from “day 0” a pregnancy is only about 38 weeks.
The reason for adding in the extra two weeks is that women usually know the date when their last menstrual period began, but have no way of knowing when they ovulated.
So during the first trimester, which is between week 1 through 13, hormones are being generated by the corpus luteum - mainly estrogen and progesterone.
By around week 9, HCG levels peak, and then begin to fall off which is a signal for the corpus luteum to finally start shriveling up.
Luckily, just as the corpus luteum is shriveling up, the placenta takes over, and specialized trophoblast cells called syncytiotrophoblast cells, make progesterone and estriol which is the most abundant type of estrogen.
The placenta also makes a bit of HCG, as well as another hormone called human placental lactogen or hPL which counters the effect of maternal insulin to help ensure that there’s plenty of glucose available in the blood for the fetus.
Many of the changes in pregnancy are directly related to the growth of the uterus.
The uterus is normally a pelvic organ, but during pregnancy it grows into the abdomen, rising to the level of the umbilicus by 20 weeks gestation and to the xiphoid process by 36 weeks.
The fundal height - which is the distance from the symphysis pubis to the top of the uterus aka the fundus is a good estimate of gestational age; for example, here at 36 weeks you might expect it to be about 36 cm, but at 20 weeks it’d be closer to 20 cm.
To accommodate the needs of mom, an enlarging uterus, and a growing fetus - as well as having some reserve for the blood loss that happens during delivery, the cardiovascular system has to expand.
Pregnancy is called a high volume state because the circulating blood volume increases by 30-50%, which means that an average woman will go from having 5 liters of blood to about 7.5 liters of blood by the third trimester.
The number of red blood cells increases a bit, but there’s a much larger increase in the plasma volume - the portion of blood that doesn’t have red blood cells.
So the hematocrit, or percentage of blood made of red blood cells, actually goes down. This is called “physiological anemia of pregnancy”.
To push this extra blood around, the heart rate goes up by about 20 beats per minute, so this increases the cardiac output.
In response to the increased workload there is a mild hypertrophy of the heart, which does go away after pregnancy.
The high volume state also explains why there’s sometimes a third heart sound or physiologic S3, as well as a split S1 which is where the mitral valve closes slightly faster than the tricuspid valve.
Finally, the blood pressure actually falls a tiny bit, even though more blood’s getting pushed through them, since progesterone causes them to dilate.
Finally, as the uterus enlarges, it pushes up on the diaphragm, nudging the heart slightly upward and shifting the heart’s point of maximum intensity - the spot where it taps against the chest wall - a little bit to the left.
The uterus also presses against the pelvic veins, causing blood to back up, leading to varicose veins as well as swelling in the lower legs and ankles.
When lying down, the uterus presses on the inferior vena cava reducing blood flow back to the right atrium and causing hypotension.
A side-lying position or placing a pillow under the hip can help avoid that.