It’s been known for centuries that women are not fertile throughout the entire menstrual cycle. The awkward term ‘periodic abstinence’ means keeping semen out of the vagina when fertilisation could occur; various methods are used to know when you are fertile so that you will know when to avoid any sexual activity that could result in pregnancy. This is often called ‘natural family planning’ because no chemicals or devices are used and the method involves an understanding of the nature of your reproductive cycle. I don’t believe that it’s really ‘natural’ to avoid sexual intercourse deliberately when there is a possibility you will conceive. Lactation is nature’s only way of delaying the next conception. I prefer the term ‘fertility awareness’.
Over the centuries every part of the cycle has been claimed to be either the fertile or the ‘safe’ time, but because most of these were wrong (as we now know) the method gained a poor reputation for reliability. It is only since the 1950s that there has been convincing evidence, based on sound research, that certain changes in women’s bodies during the menstrual cycle reflect what’s happening in our ovaries, particularly hormone production and ovulation. We can observe and interpret these changes to predict and identify when we are fertile in the cycle. But first we must know some important facts concerning sperm, ova and the effects of ovarian hormones on body temperature, cervical mucus and the cervix.
The quality of sperm vary in regard to the time they will survive, their ability to swim (motility) and their ability to fertilise the ovum. This variation exists between men and between individual sperm within the same man’s semen. From the point of view of preventing pregnancy we must assume that all semen contains some sperm of the ‘very best’ quality.
As you can imagine, it is very difficult to study sperm survival after an ejaculation into a woman’s genital tract. Most of our knowledge comes from post-coital tests and from IVF (in-vitro fertilisation) studies. The important factors that seem to influence sperm survival after ejaculation are:
• conditions in the uterus and tubes. Without fertile cervical mucus sperm cannot pass through the cervical canal and die quickly in the vagina. Studies of sperm survival in women wearing cervical barriers (and thus there is no cervical mucus in the vagina) reveal that no live sperm have been found in the vagina three hours after ejaculation and in most cases all were dead after one hour.
Once sperm have entered fertile cervical mucus they can survive on average three to four days. However, the fact that women have conceived when they have had no coitus for seven days (in one case nine days) before the known time of ovulation shows that some vigorous sperm can survive for longer in the right conditions.
Most sperm are motile when they are ejaculated but must undergo further changes before they are able, to fertilise the ovum. These changes normally occur they pass through the cervical, uterine and tubal fluids that have been produced under the influence of oestrogen. Note, however, that in these days of assisted conception technologies, ejaculated sperm are also able to fertilise eggs in the test tube.
In humans it would seem that after ejaculation a reservoir of sperm can remain in fertile cervical mucus for some days, from where they move in relays through the uterus and then on to the tube. This explains how fertilisation can sometimes happen up to a week after the last sexual intercourse. The average ejaculate contains around 200 million sperm. Most of the ejaculate is wasted in the vagina, but if 1 per cent of it enters fertile cervical mucus and if 1 per cent of the sperm in this fraction were и the ‘best quality’, a pregnancy could result up to a week later.