Obstetrics 1: Physiology of Pregnancy Portal

The uterus rises out of the pelvis at 12 weeks gestation (3 months, the beginning of the second trimester).
The uterus reaches the umbilicus at 20 weeks gestation (5 months).
The head of the fetus usually becomes fixed in the pelvis at about 24 weeks (6 months, the beginning of the third trimester).
The mother’s heart rate increases throughout pregnancy, and during the third trimester it is increased by about 15 to 20 bpm.
The mother’s BP falls 5 to 15 torr in systolic and diastolic pressures during the second trimester. BP returns to normal levels near term.
Blood volume increases by about 48% near term. This increase is predominantly in plasma volume, but there is also some increase in red cell mass.
Hematocrit falls and a 31% to 35% hematocrit is normal near term.
WBC increases to 20,000/mm³ near term.
The tidal volume of breathing increases resulting in hypocapnea with a PaCO₂ of 30 torr. Respiratory rate is unchanged. Arterial pH remains normal.
Renal perfusion is increased with a resulting fall of serum urea nitrogen (BUN) and creatinine to one-half of pre-pregnancy levels.
The symphysis pubis widens to 4 to 8 mm and the sacroiliac joint spaces widen.
The supine hypotensive syndrome results from the weight of the uterus on the vena cava when a pregnant woman lies on her back. This will be relieved if she lies on her left side.
The placenta is exquisitely sensitive to catecholamines with resulting vascoconstriction with only slight changes in the mother’s blood volume.