For normal fertility, sperm in the vagina must swim up through the cervix, uterus and fallopian tubes to meet an ovulated egg that has been carried from the surface of the ovary to the mid-part of the fallopian tube. The embryo that results (strictly speaking it’s still a “pre-embryo”) develops for three days in the fallopian tube. It then travels to the uterus, where it floats and develops for another three or four days, before attaching to and implanting within the lining of the uterus – thus establishing pregnancy. A few days before the period is missed, a blood pregnancy test will be positive.
It’s at about this time of implantation, a week after fertilization, that the first few cells in the center of the “embryo” actually differentiate into what will be the fetus; all other cells (the majority of cells at this stage) go to form the placenta.
The general reasons a couple may be relatively infertile (with a reduced monthly chance of conception) or completely infertile (with no chance of conception, sometimes called sterility) are:
Problems with ovulation (the release of the egg from the ovary). There will be complete infertility if periods are absent (“amenorrhoea”), although many such patients are treatable with hormones or drugs. If there are no responsive eggs in the ovaries (“ovarian failure”) egg or embryo donations are required.
Problems with sperm production (often not treatable, except by assisted conception). There will be complete infertility if there are no sperm in the ejaculate (“azoospermia”) though problems due to blockages are now treatable with assisted conception.
A blockage between the vagina and the ovary, preventing fertilization (the most common site of blockage is the fallopian tubes – sometimes treatable with microsurgery, or assisted conception).
Endometriosis, a common condition in which tissue like the lining of the uterus, the “endometrium” grows outside the uterus, disturbing a number of events essential to conception and implantation of the embryo in the uterus; treatment can be medical, surgical or with assisted conception; infertility is usually relative rather than complete.
