Assessment and management of women during postnatal period...

The puerperium covers the six-week period following birth, during which time the various changes that occurred during pregnancy revert to the non-pregnant state. Physiological changes during this time include:

The cardiovascular system reverts to normal during the first two weeks. The extra load on the heart from the extra volume of blood disappears by the second week.
The vaginal wall is initially swollen, bluish and pouting but rapidly regains its tone, although remaining fragile for 1-2 weeks. Perineal oedema may persist for some days.
After delivery of the placenta, the uterus is the size of a 20-week pregnancy; however, it reduces in size on abdominal examination by one finger-breadth each day, such that on the 12th day it cannot be palpated. By the end of the puerperium it is only slightly larger than pre-pregnancy.
For the first 3-4 days, lochia comprises mainly blood and remnants of trophoblastic tissue. During days 3-12 the colour is reddish-brown but then changes to yellow. Occasionally, lochia may become red again for a few days, due to thrombi from the ends of vessels.