Fortunately, conceiving a child is easy for many couples. Potential parents don’t usually have to think about the many microscopic interactions between the sperm and egg that are necessary to make a baby. The average couple will become pregnant by the third month of trying, and 90% will be pregnant within one year. Unfortunately, not everyone becomes a parent so easily. One out of 10 couples has trouble conceiving despite trying for an entire year. These couples are advised to seek medical attention to attempt to uncover what part of the process is not functioning normally. Understanding the nuts and bolts of human fertility becomes very important in these cases. The entire process of producing sperm and eggs and allowing them to meet, creating an embryo, and allowing it to grow in the womb is delicately balanced in fertile humans.
The Egg in the Human Body
A woman is born with about 2 million eggs in her ovaries. She can never make any new eggs after that time. By the time she reaches puberty, only about 200,000 eggs remain. Everyday a number of eggs wither away. Beginning at puberty, one egg per month will mature to the point where it can be fertilized. The egg is contained in a fluid-filled egg sac called a follicle. The follicle grows to about one inch in diameter before it ruptures – releasing the egg in a process called ovulation. Ovulation typically occurs around day 14 of the menstrual cycle if the woman has a typical 28 day cycle. A woman may get clues that she will be ovulating within the next 3 days by noticing an increase in the volume of cervical mucus that become stretchable (like uncooked chicken egg white).
Sperm are produced in the male’s testicle beginning at puberty. New sperm are constantly being made throughout the man’s life. It takes about 2 1/2 months for a sperm to be made from start to finish. The mature sperm are stored in the epididymis (a tube near the testicle) until ejaculation – at which time they are transported out the end of the penis, after mixing with secretions from the prostate and other glands, to constitute what is called semen. Normally there are at least 30 million moving sperm in the ejaculate. The sperm in the epididymis are usually sufficiently replenished by 24-48 hours after ejaculation.
The Fertilization Process
One sperm penetrates the shell around the egg. Once this happens, the shell alters itself to prevent any other sperm from entering. Now the DNA (the genetic material contained within the chromosomes) from the sperm joins with the DNA of the egg. Together they make up the full complement of genes needed to create a human. The process is called fertilization.
The Meeting of Sperm and Egg
When the semen is deposited in the vagina, the sperm swim into the mucus that resides in the woman’s cervix. The cervix is the mouth of the uterus or womb. The rest of the semen then leaks out of the vagina. The sperm can survive in the mucus for up to 3 days; however, they lose a bit of their vigor after 48 hours. This is why conception is more likely to occur if sexual relations occur at least every other day during the fertile week. Meanwhile, a proportion of the sperm are swimming into the uterus and traveling up both fallopian tubes where they await the arrival of the woman’s egg. The ovulated egg is picked up by the fimbriated end of the fallopian tube, which resembles a vacuum sweeper. The fallopian tube serves as a conduit for the egg to travel from the ovary to the uterus. The fallopian tube is also the site where the egg encounters the marauding gang of sperm.
The fertilized egg (also called an embryo) must begin dividing into many cells as it grows. Typically the embryo reaches the interior cavity of the uterus (womb) 4 days after fertilization occurs. The lining of the uterus has been made hospitable for the embryo by 2 hormones (estrogen and progesterone) produced by the ovary. If the embryo is sufficiently healthy to implant in the lining of the uterus and keep growing, then the woman misses her period. A pregnancy test is reliably positive 14 days after ovulation.
What can go wrong?
Obviously there are many steps in the process where things can go awry. At least 5% of the men in the population have insufficient numbers of moving sperm in their ejaculate. This can be due to a problem in the manufacturing of the sperm or due to an obstruction in the tube that transports the sperm to the outside world. Depending on the severity of the sperm deficiency, pregnancy can be achieved by inseminating the sperm directly into the uterus or by the use of In-Vitro Fertilization. In the most severe cases, where no sperm can be found even in the testicle, then donated sperm is an option. Perhaps the most common problem encountered on the woman’s side is incomplete maturation of an egg in most of her monthly cycles.
The delicate balance of hormones that govern the egg maturation can be restored while using various fertility pills such as clomiphene or letrozole or injectable gonadotropins (such as Gonal-F, Follistim or Bravelle). If a woman has run out of eggs (premature menopause) then the only chance for her to carry a pregnancy is to use donated eggs. If the fallopian tubes are blocked, then sperm and egg can’t meet. This problem can be overcome either by surgical correction of the tubes or the use of In-Vitro Fertilization. There are conditions that can cause more subtle defects in the ability of the egg to reach the tube. Scar tissue surrounding the ovary is one example and endometriosis – which sets up a toxic reaction to the ovulated egg – is another. Minimally invasive surgery such as laparoscopy is used to vaporize the endometriosis and to cut any scar tissue. Armed with this basic knowledge of human reproduction, potential parents who are having trouble making a baby can better negotiate the emotional journey into the realm of medical treatment to achieve their dream of building a family.