The Sperm's Long Journey. A man may ejaculate 40 million to150 million sperm, which start swimming upstream toward the fallopian tubes on their mission to fertilize an egg. Fast-swimming sperm can reach the egg in a half an hour, while others may take days. The sperm can live up to 48-72 hours. Only a few hundred will even come close to the egg, because of the many natural barriers that exist in a woman's body.
9.4 Giving birth. Normal delivery. Children that are born between the 36th and 39th week (38 and 41 weeks after the LMP) are considered as being normal deliveries. Statistically, only 2/3 of all children are born within the 3 weeks around the calculated date of birth and around 80 % within a month around the predicted date of birth.
A normal pregnancy lasts for 40 weeks. A perfect normal full term pregnancy is 40 weeks. However, it is normal to go into labor at 37-38 weeks. How Many Weeks Is Pregnancy. A full term pregnancy is 40 weeks, but it is safe to delivery a baby between 38-42 without it having any health issue. 40. 40 weeks from the first day of your last period.  as said by brain child is correct. that is how the due date is calculated.