REPRODUCTIVE HEALTH EDUCATION – Basic Fertility Physiology
Author: J. Adeghe PhD, FRCOG. Consultant – Gynaecology & Assisted Reproduction
This is about knowledge of some basic fertility facts to boost the chance of achieving a pregnancy. Conception takes place when a woman’s egg is fertilised by a man’s sperm. For this process to be successful both egg and sperm have to be of good quality, the egg has to be released (ovulation) into the fallopian tube, the fallopian tube has to healthy and patent (open), the sperm has to swim from the vagina, through the cervix and into the uterine cavity and then into the fallopian tubes. The sperm has to penetrate into the egg through the egg shell called Zona Pellucida. Finally, the fertilised egg develops into an embryo which implant in the uterine lining (endometrium). It is evident from the above that there are many steps in the fertility process. Generally, the more complex a process is, the higher the risk of malfunction. This applies to human reproduction.
Human reproduction is inefficient. The chance of a normal couple (with no fertility problems) becoming pregnant on a month to month basis is only 25%. A normal semen sample produced during sexual intercourse contains an average of 22 million sperm with at least 40% swimming well. Yet it takes multiple sexual intercourse (ideally 2-3 times a week) over several months (average of 6 months) before one sperm fertilises one egg to achieve a pregnancy. Compare this with reproduction in some animal species where a single act of sexual intercourse results in the birth of multiple offsprings!.
Normal female monthly cycle is between 27 to 32 days. An egg is released around mid-cycle which is 13 to 16 days from the start of the last menstruation. The egg is fertilised within 12 to 24 hours of being released.
At the risk of stating the obvious, the biology (basis) of male and female fertility are different. A woman is born with a fixed number of eggs in her ovaries (about half a million) akin to having a set amount of money in a deposit bank account. The difference though, is that while money in a bank account may yield interest and increase your deposit, the number of eggs in the ovaries does not increase, on the contrary it actually declines in quantity and quality with increasing age. This is the reason for the decline in female fertility with age. This decline in fertility kicks in in the mid-30s and becomes significant from 40 years age. Pregnancy rates are very low after 44 years even with fertility treatment.
The male fertility system is centred within the testes where spermatozoa (sperm cells) are produced. The production and maturation of sperm within the testes (spermatogenesis) takes about 2 to 3 months and require a lower temperature, a few degrees below normal body temperature. This is why the testes are positioned in the scrotum, and not inside the body. The essence of spermatogenesis is that fresh sperm cells are produced every 2 to 3 months. Compare this to females where a 40 year old woman has eggs that that have been present from birth, meaning that the eggs are 40 years old! I say this to emphasise the difference between men and women with ‘reproductive design’. It is not only that natural conception is less likely in women over 40years, the risk of miscarriage is higher and the risk of chromosomal abnormalities in the offspring is also higher. These fertility facts underline the disadvantages of undue delay in starting a family.