Conception occurs when the father's sperm fertilizes the mother's egg. After several days, implantation of the fertilized egg in the uterus proceeds. During the next several weeks, most of the major organs of the baby develop. The estimated due date is typically 40 weeks from the woman's last menstrual period. Therefore, the estimated date of delivery is actually 38 weeks from the date of conception.
Most physicians prefer to keep track of pregnancy in terms of weeks rather than months.
The first, second and third trimesters each represent a third of the total duration of pregnancy. Keep in mind that the estimated date of delivery is just that- estimated . Delivery a week to 10 days before or after the estimated date of delivery is normal. A variety of different physiological changes occur in the woman during pregnancy. Her heart rate increases as does the amount of circulating blood.