During coitus (sexual intercourse) between a male and a female, semen is released into the vagina and transported through the uterus into the fallopian tube. Although many factors contribute to whether or not a single act of intercourse will result in pregnancy, most important is whether or not a sperm cell will "meet" an ovum in the fallopian tube (fertilization). Fertilization can only occur if intercourse takes place before the time of ovulation that usually occurs "mid-cycle", or about 14 days before the woman's next menstrual period. At the time of ovulation, the ovum is released from the ovary and transported in the fallopian tube where it remains for about 24-48 hours. Pregnancy is most likely to occur if fresh semen is present when ovulation occurs.
Sperm cells remain viable within the female reproductive tract for about 72 hours. Only a single sperm cell is needed to fertilize the ovum, even though the average ejaculation contains approximately 300 million sperm.
During fertilization, the sperm enters the cell membrane of the ovum so the nuclei of the sperm and egg cells combine to form a zygote. The zygote will remain in the fallopian tube for approximately three days before it travels to the uterus where it will remain for approximately four to five days before implantation into the uterine lining.
An ectopic pregnancy is when a fertilized egg becomes implanted outside the uterus, most often in the fallopian tubes. Ectopic pregnancies are often the result of fallopian tube scarring caused by untreated sexually transmitted infections, which impedes the transport of the zygote from the fallopian tubes to the uterus. If left undiagnosed or untreated, the zygote grows in the fallopian tube, and may result in fallopian rupture. Without medical or surgical intervention, ectopic pregnancy usually results in infertility, severe infection, or death. Ectopic pregnancy can be difficult to diagnose, as it shares many symptoms with uterine pregnancy. However, Some symptoms of ectopic pregnancy include pain in the vagina, abdomen, or lower back, vaginal bleeding or spotting, dizziness, fainting, and low blood pressure.
Prior to fertilization, the sperm and egg cells have only half the number of chromosomes (the genetic determinants of heredity) of other body cells because they go through a process of cell division called meiosis, which reduces the number of chromosomes from 46 to 23. The 23 chromosomes from the sperm cell combine with the 23 chromosomes from the ovum to form a single cell (zygote) consisting of 46 chromosomes.
Sex of the offspring is determined by whether the sperm contains an X or a Y sex chromosome. Female ova always carry an X chromosome. If the sperm cell contains a Y chromosome, the offspring will be a male (XY pair). If it contains an X chromosome, the offspring will be a female (XX pair). Thus, the sperm biologically determines sex.