Female age is the most important factor affecting fertility. Women are born with all the eggs they will ever have and the number of eggs available decreases each day from birth onwards. In young women the decline is fairly gradual (only a few eggs are ‘lost’ each day), but as women approach their mid to late 30s, the decrease gets much steeper (many more eggs are ‘lost’ each day). In addition to this decrease in the number of eggs available, the quality of the eggs also declines as women get older. This reduction in both the quantity and quality of available eggs means that older women are less likely to get pregnant and, if they do get pregnant, they are more likely to have a miscarriage.
Male fertility may also decrease with age although to a much lesser degree.
Couples are more likely to get pregnant if they have previously achieved a pregnancy together (irrespective of whether or not that pregnancy resulted in the birth of a baby) compared to couples that have never been pregnant.
Duration of subfertility
The longer couples have been trying to get pregnant, the less likely they are to be successful. If a couple have been trying to get pregnant for less than 3 years they are almost twice as likely to get pregnant than couples who have been trying for more than 3 years.
Timing and Frequency of Sexual Intercourse
Most women have a 28-day menstrual cycle: this means there are 28 days between the start of one period and the start of the next period. Ovulation (when an egg is released from the ovary) occurs on day 14 of a 28-day cycle. Whether or not that egg gets fertilised by a sperm depends on the day(s) in the cycle in which intercourse occurs. The chance of getting pregnant is low at the beginning of the cycle and starts to increase from about day 8 onwards. Women are most likely to get pregnant if they have sex 2 days before they ovulate (i.e. on day 12 of 28-day cycle).
Once ovulation has occurred, the chance of getting pregnant decreases dramatically: sperm need to be present in the female genital tract prior to ovulation to maximise the chances of getting pregnant. The reason for this is that once ovulation has occurred, levels of the hormone progesterone increase and this causes cervical mucus to become thick and sticky which prevents sperm from being able to swim through it to get to the egg to fertilise.
Couples that have regular sex 2-3 times a week are most likely to get pregnant because this frequency ensures that a good volume of fresh sperm will be present in the female reproductive tract at the time of ovulation. If couples are only having sex once a week, the chance of getting pregnant is less because there will be less fresh sperm present in the female genital tract at the time of ovulation.
Overweight women who have irregular periods are less likely to release an egg each month (ovulate) than women with regular periods. This means the chances of getting pregnant are reduced. Losing weight, even as little as 5-10% of the total body weight, may restore a regular menstrual cycle thereby increasing the chance of getting pregnant.
Women who are overweight take longer to get pregnant than women who are not, even if their periods are regular. Being overweight is also associated with an increased risk of miscarriage and of problems during pregnancy and delivery. Losing weight has been shown to improve fertility and increase the chance of getting pregnant. No one diet has been shown to be better than any other but group exercise programmes that involve both exercise and dietary advice leads to more pregnancies than weight loss advice alone.
Men who are overweight may have suboptimal sperm and therefore reduced fertility.
Being underweight can also affect female fertility as it may mean that an egg is not released each month. Being underweight is also associated with problems during pregnancy. Gaining weight is likely to improve fertility.
Women who smoke are 3 times more likely to experience a delay in getting pregnant than non-smokers. Even passive smoking can be harmful. Smoking reduces a woman’s ovarian reserve (so her ovaries will have fewer eggs in them than a woman of the same age who does not smoke) and damages the cilia inside the fallopian tube (which are important for transporting the egg and/or embryo along the fallopian tube into the uterus). In men, smoking may reduce sperm quantity and quality.
There is no clear association between caffeine consumption and infertility
Some studies report that drinking more than 5 units of alcohol a week may reduce female fertility but others state that low to moderate alcohol consumption may be associated with higher pregnancy rates than non-drinkers. Once pregnant, excessive alcohol consumption may lead to birth defects and developmental delay. The Royal College of Obstetricians and Gynaecologists and the Department of Health recommend that women trying to get pregnant should avoid alcohol because there is no ‘safe’ limit.
In men, excessive alcohol may lead to difficulties maintaining an erection, impaired ejaculation and reduced sperm quality.
Over-the-counter and recreational drugs
Non-steroidal anti-inflammatory drugs such as ibuprofen can interfere with ovulation. Aspirin may interfere with implantation. Recreational drugs such as marijuana and cocaine may interfere with ovulation and/or the function of the fallopian tube. The fallopian tube is important for transporting the egg from the ovary where it is released, to the womb (uterus) where an embryo will hopefully implant. Fertilisation occurs in the fallopian tube. Anabolic steroids, which are abused by some body-builders, inhibit the production of sperm and this may be permanent even if the drug is stopped.
Some women may have medical conditions that can affect their fertility. These may or may not be known about when starting to try for a family. Some of these conditions may be more general, for example thyroid disease and vitamin D deficiency whilst others may be more specific, for example, polycystic ovary syndrome and endometriosis.