The conception date is the date that the sperm and egg fertilize together and attach themselves to the uterus. With humans, this can occur anytime between 24 hours and 72 hours after a male and a female have had sexual intercourse. The due date of a baby is normally calculated from the first day of the last period before pregnancy. This normally means that women are actually 2 weeks further along in pregnancy than they may think. The doctors will work out the date of conception in this way to enable them to monitor the development of the baby more closely. All mammals rely on internal fertilization and the sperm cells move towards the ovum where they meet and bind together. The egg only requires one single sperm cell to change its cell membrane and this prevents it interacting with other sperm. Only when the egg, which has interacted with the sperm, reaches the uterus has the pregnancy begun. The embryo can sometimes implant itself into other tissues and this is the cause of an ectopic pregnancy, which can be fatal to both the baby and the mother. 15 to 16 days after fertilization, the genetic code of the father is fully interactive with the development of the embryo, and it is in the stages up to this that multiple births are created. Some animals will fertilize externally. An example of an animal whose eggs fertilize externally is the chicken. There are some advantages to external fertilization and these include a minimal transmission of bodily fluids and a decrease in the risk of transmitting diseases. Disadvantages include the lack of protection of the egg and less chance of individual egg fertilization.
