Although we organize pregnancy into three trimesters, the length of a menstrual cycle is the basis for an entire pregnancy - and each woman's cycle varies. On average, a menstrual cycle is 28 days from beginning to end. Pregnancy averages about 280 days which translates nicely into 40 weeks or 10 lunar months. Dividing 280 days by 3 (the number of trimesters) equals about 12.666 weeks for each trimester. But trimesters do not truly reflect embryo to fetal growth.
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Most obstetric professionals follow a 40-week timetable based on the average 28 day menstrual cycle. But doctors also track specific parameters such as the distance from the mother's pubic bone to the height of the uterus, or use ultrasound images to measure the fetal head diameter to judge normal fetal growth and predict fetal age. It is important to visit a medical professional during pregnancy to know if normal growth is proceeding.
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The timing of infant development is better illustrated in this chart which does not confine itself to the three trimester system.
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