The date of conception is the day that the sperm fertilizes the ovum and pregnancy begins. In a normal cycle, this occurs two weeks after the first day of the last menstrual period.
About two weeks prior to the beginning of the next menstrual cycle, an egg is released from the ovary. The egg travels to the fallopian tube where it remains until it is fertilized by a sperm cell.
The fertilized egg travels through the fallopian tube and into the uterus. After about three days it will enter the uterus and implant itself into the uterine wall.
Implantation bleeding can occur when the newly fertilized egg attaches to the uterine lining. This occurs about 6 to 12 days following ovulation and may accompany light cramping.
Calculating Conception Date
Ovulation usually occurs about two weeks prior to the start of your menstrual cycle. Knowing the length of her cycles and the date of the last menstrual period, a woman can calculate the date of conception by counting backwards two weeks from the day that her next cycle would have begun in the absence of pregnancy.
Length of Pregnancy
Doctors and midwives begin counting the weeks of pregnancy starting with the first day of the last menstrual period. At the time of conception, a mother is already two weeks pregnant according to this schedule. A normal pregnancy lasts about 40 weeks from the date of the last menstrual period or 38 weeks from conception.