The condition of carrying one or more embryos in the uterus is called Pregnancy or Gestation. Human pregnancy lasts for average 266 days from fertilization of the egg or 280 days from the start of the last menstrual cycle.
Human pregnancy can be divided for convenience into three trimesters of about three months each. The first trimester is the time of most radical change for both the mother and the embryo. During its first 2-4 weeks of development, the embryo obtains nutrients directly from the endomentrium.
The first trimester is the main period of organogenesis and the development of body organs. At 8 weeks, all the major structures of the adult are present in the rudimentary form and the embryo is called a foetus. It is only 5 cms long.
Meanwhile, the mother also undergoes rapid changes. High levels of progesterone initiate changes in her reproductive system, the maternal part of placenta grows, the uterus becomes larger, ovulation and menstrual cycle stops.
During the second trimester, the uterus grows enough for the pregnancy to become obvious. The foetus is very active and grows to about 30 cms long. Hormone levels stabilize as HCG declines, the corpus luteum deteriorates and the placenta completely takes over the production of progesterone which maintains the pregnancy.
During the third (final) trimester the foetus grows to about 3-4 kg in weight and 50 cms in length. As the foetus grows, the uterus expands around it, the mother's abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages and strain in the back muscles.