Pregnant women produce more Oestrogen in a day than non-pregnant women do in 3 years!
The placenta is an oestrogen making machine and ramps up its production of this hormone to this tremendous level towards the end of pregnancy.
A woman will make more oestrogen in one pregnancy than she will during the rest of her non-pregnant life.
Oestrogen is important during pregnancy because of its proliferative effects on tissues and is responsible for enlargement of the uterus, breasts (and their ducts) and genitalia of the mother.
http://www.greenvillemidwiferycare.com/common-discomforts-during-pregnancy.php