The uterine lining thickens throughout the month until a woman has her period and the lining is shed, according to BellyBelly. If a woman is pregnant, however, the uterine lining continues to thicken in early pregnancy in order to protect the baby.
The thickening of the uterine lining is controlled by the hormone progesterone, as stated by BellyBelly. During pregnancy, the ovaries continue to produce this hormone for seven weeks, until the placenta takes over this process. Progesterone is what causes many of the early side affects in pregnancy such as nausea, but it also prepares the body for the baby by helping the mother add body fat.