Fertilization and Implantation   


How Does Pregnancy Happen?


About halfway through the female menstrual cycle, an oocyte is released (ovulated) from the ovary  into the nearby fallopian tube. After vaginal intercourse, sperm is ejaculated into the female body via the vagina. This sperm can quickly swim up through the cervix (opening of the womb or uterus) and then continues its journey through the uterus into the fallopian tubes, arriving at the egg in less than 20 minutes! Here, the sperm wriggles its way through the oocyte capsule and combines its DNA with the that of the egg in a process known as fertilization. (Below are electron micrographs of the human fertilization process.)


The fertilization process is complete approximately 18 hours after the sperm has found the egg. Once the sperm and oocyte have come together, the pairing that results is called a zygote. This zygote then travels to the uterus, pushed along by the motion of sweeping hair-like structures (cilia) lining the fallopian tubes. The passage from the fallopian tube to the uterus takes about one day. During the time the zygote, or new embryo travels, it also simultaneously matures. About 5 or 6 days after fertilization, when the embryo has achieved a more advanced stage known as a blastocyst, the embryo sticks itself to the uterus. Sometimes the fertilized egg has trouble traveling down the fallopian tube and gets stuck in the tube instead of in the uterus. This is a dangerous and potentially life-threatening condition known as an ectopic (or tubal) pregnancy, which requires the immediate attention of a health care provider.



Implantation occurs when the embryo attaches itself to the lining of the uterus, approximately one week after ovulation. Once the embryo is firmly attached to the mother, it begins to develop rapidly and takes needed nutrients from the motherís blood supply. The process of implantation can be prevented by regular use of birth control pills or by taking emergency contraception pills. Once the embryo has attached to the uterus, these methods of preventing pregnancy no longer work.

Click the Pregnancy animation tab on the right to learn more.