The pregnancy period of approximately nine months (280 days) is divided into three trimesters of three months each.
1. First Trimester : (From fertilization to 12th week)
- During first trimester there are radical changes in the body of mother as well as in the embryo.
- The embryo receives nutrients in the first 2-4 weeks directly from the endometrium.
- It is the main period of organogenesis and the development of body organs.
- By the end of eight weeks, the major structures found in the adult are formed in the embryo in a rudimentary form. It is now called foetus and is about 3 cm long.
- Arms, hands, fingers, feet, toes, CNS, excretory and circulatory system including heart are formed and begins to work.
- Progesterone level becomes high and menstrual cycle is suspended till the end of pregnancy.
- At the end of first trimester foetus is about 7-10 cm long.
- The maternal part of placenta grows, the uterus becomes larger. In this period, the mother experiences morning sickness, (nausea, vomiting, mood swings, etc.)
2. Second Trimester: (From 13th to 26th week)
- The foetus is very active and grows to about 30 cm.
- The uterus grows enough for the pregnancy to become obvious.
- Hormone levels stabilize as hCG declines, the corpus luteum deteriorates and the placenta completely takes over the production of progesterone which maintains the pregnancy.
- Head has hair, eyebrows and eyelashes appear, pinnae are distinct. Baby’s movement can be easily felt by the mother.
- The baby reaches half the size of a new born.
3. Third Trimester: (From 27th week till the parturition)
- Foetus grows to about 50 cm in length and about 3-4 kg in weight.
- 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.
- At the end of third trimester the foetus becomes fully developed and ready for parturition.
