Fertility and Reproductive Ageing

Fertility is sometimes discussed as though a person is either fertile or infertile. In reality, there is a chance of conception during each cycle, and that chance changes with age, health, timing and reproductive anatomy.

Fertility is a probability Fertility is sometimes discussed as though a person is either fertile or infertile. In reality, there is a chance of conception during each cycle. That chance changes with age, health, timing and reproductive anatomy. A person can have regular periods and still experience difficulty conceiving. Another person with an identified fertility factor may still conceive without treatment. A fertility diagnosis therefore describes probability and clinical findings. It does not predict one individual outcome with certainty. What must happen for conception Several biological steps must align for a pregnancy to begin. An ovary usually needs to release an egg through ovulation. Sperm must be produced, ejaculated or introduced into the reproductive tract, and able to move towards the egg. At least one fallopian tube must usually allow the egg and sperm to meet. Fertilisation most often occurs within a fallopian tube. The resulting early embryo must continue developing while travelling towards the uterus. The uterine lining must then support implantation, when the embryo attaches and begins forming a connection with the body. Difficulty at any stage can reduce the chance of pregnancy. More than one factor may be present. Timing and the fertile window An egg remains capable of fertilisation for a relatively short period after ovulation. Sperm can remain capable of fertilisation for several days within the reproductive tract. The fertile window therefore includes the days before ovulation and the day around ovulation itself. Ovulation does not always happen on the same calendar day. Illness, stress, weight change and naturally variable cycles can change its timing. Having vaginal sex every two to three days generally covers the fertile window without requiring exact prediction. Cycle tracking tools can estimate timing, but they do not confirm that ovulation has occurred or guarantee conception. Reproductive ageing in ovaries The ovaries contain their lifetime supply of immature eggs before birth. The number of remaining eggs decreases naturally throughout life. This is called a decline in ovarian reserve. Age also affects the average developmental quality of the eggs that remain. Eggs must divide their chromosomes accurately during maturation and fertilisation. Errors become more common as eggs age. This contributes to lower conception rates and higher miscarriage rates with increasing maternal age. It also increases the chance of some chromosomal conditions. Age changes probability, not certainty. Fertility usually declines before periods stop. Regular cycles therefore do not prove that fertility remains unchanged. The decline becomes more clinically important during the later reproductive years, but there is no single age when natural conception becomes impossible for everyone. Menopause and natural conception The menopause is reached after ovarian activity has declined enough for menstrual periods to stop permanently. Natural conception is no longer expected after menopause because regular ovulation has ended. The transition is gradual. During perimenopause, ovulation becomes less predictable but pregnancy may still occur. Contraception may therefore remain necessary even when periods have become irregular. Reproductive ageing in testes The testes continue producing sperm throughout much of adult life. There is no direct male equivalent of menopause that stops fertility at one predictable age. However, reproductive ageing can gradually affect semen volume, sperm movement, sperm DNA and sexual function. Health conditions and medicines also become more common with age. These may affect erections, ejaculation or sperm production. The effect of paternal age is usually more gradual than the effect of ovarian ageing. Older age does not automatically mean infertility. It may reduce probability and alter some pregnancy risks. Factors other than age Age is important, but it is not the only influence on fertility. Ovulation can be affected by polycystic ovary syndrome, thyroid disease, high prolactin levels, very low energy availability or substantial weight change. Fallopian tubes can be damaged by previous pelvic infection, endometriosis, ectopic pregnancy or surgery. The uterus may be affected by some fibroids, polyps, scarring or developmental differences. Sperm production can be affected by testicular injury, infection, genetic conditions, heat exposure, medicines, anabolic steroids or cancer treatment. Smoking can reduce fertility in people producing eggs or sperm. Some sexually transmitted infections can also damage reproductive structures. In some people, standard investigations do not identify one cause. This is called unexplained infertility. When difficulty conceiving becomes infertility Infertility is a clinical term used when the time trying to conceive justifies formal investigation and possible treatment. Current NICE guidance recommends further assessment of both partners after one year of regular unprotected vaginal intercourse without conception. Referral should be offered earlier when the person trying to become pregnant is aged 36 or over. Earlier assessment is also appropriate when either person has a known or suspected fertility factor, such as very irregular periods, previous pelvic infection, endometriosis, testicular problems or treatment that may damage eggs or sperm. People using artificial insemination should usually be assessed after six unsuccessful cycles when no earlier concern exists. Eligibility for NHS funded treatment can vary locally. This is separate from whether a clinical assessment is appropriate. Both partners should be considered Fertility assessment should not focus only on the person hoping to carry the pregnancy. Egg, sperm, reproductive tract and combined factors can all contribute. Assessment of both partners at the same time can reduce delay and avoid unfair assumptions. Questions may include menstrual cycles, previous pregnancies, sexual function, infections, surgery, medicines and occupational exposures. The discussion should also recognise donor conception, same sex relationships, single parents and people who cannot have vaginal intercourse. Semen analysis A semen analysis assesses several features of an ejaculated semen sample, including semen volume, sperm concentration, movement, survival and appearance. A result outside a reference range does not prove permanent infertility. Results vary naturally and may change after fever, illness or incomplete sample collection. An abnormal result is often repeated. Further assessment depends on the degree and pattern of the abnormality. A result within reference ranges does not guarantee conception. It is one part of a wider assessment. Assessing ovulation Cycle history provides useful information. Regular monthly periods make regular ovulation more likely, but they do not prove it. A progesterone blood test taken at the correct phase of the cycle can provide evidence that ovulation has occurred. The appropriate timing depends on cycle length. It is not always correctly described as a fixed "day 21" test. Other hormone tests may be used when cycles are irregular or when a specific endocrine condition is suspected. Ovarian reserve and AMH Anti Müllerian hormone, usually shortened to AMH, is produced by cells surrounding small ovarian follicles. AMH can help estimate how the ovaries may respond to medicines used during assisted conception. It does not directly measure egg quality. It also does not reliably predict natural conception for an individual. A lower AMH result does not mean pregnancy is impossible. A higher result does not guarantee pregnancy. Current NICE guidance advises against using AMH to predict the chance of spontaneous conception. Tubes and uterus Testing the fallopian tubes may involve ultrasound, an X ray contrast test or keyhole surgery. The chosen test depends on past health problems and the likelihood of pelvic disease. Ultrasound can assess the uterus and ovaries and can identify some fibroids, ovarian cysts or anatomical differences. A normal scan does not assess every fertility process. It cannot confirm egg quality or guarantee open tubes unless the specific test evaluates them. Assisted conception Assisted conception includes treatments that help eggs and sperm meet or support early embryo development. Intrauterine insemination places prepared sperm into the uterus around ovulation. In vitro fertilisation, or IVF, involves collecting eggs and fertilising them in a laboratory. An embryo may then be transferred into the uterus. IVF does not guarantee implantation or live birth. Intracytoplasmic sperm injection, called ICSI, involves injecting one sperm directly into an egg. ICSI can help in selected sperm related situations. It does not correct every cause of failed fertilisation or pregnancy loss. Treatment success remains strongly influenced by the age of the egg. Using donor eggs can change age related probabilities because outcome is closely related to the donor egg's age. Fertility preservation Some medical treatments can damage eggs, sperm or reproductive organs. Chemotherapy, pelvic radiotherapy and surgery involving the ovaries or testes can affect future fertility. Fertility preservation options may include freezing sperm, eggs, embryos or ovarian tissue. These options must be discussed before treatment whenever time and clinical circumstances allow. Freezing reproductive cells preserves their age at collection. It does not guarantee a future pregnancy. The emotional impact Difficulty conceiving can affect mood, relationships, sexual wellbeing and identity. Repeated testing may create uncertainty rather than one clear answer. People should receive information in a way that respects privacy, family structure, culture and reproductive choices. Psychological support is a legitimate part of fertility care, not an indication that the problem is imagined.

Fertility depends on several linked processes, and reproductive ageing changes probability rather than creating one universal expiry date.