Understanding fertility

How conception works

Four things have to go right in the same month. An egg has to be released. Sperm has to reach it. The two have to meet in a fallopian tube and fertilise. And the resulting embryo has to reach the uterus and implant in a lining that is ready for it.

A problem at any one of those points reduces the chance of conception, and fertility investigations are essentially a way of asking which of the four is not working.

What is normal, month to month

A healthy couple in their late twenties, with everything working, has roughly a one-in-five chance of conceiving in any given month. That figure surprises people. It means that not conceiving in three or four months is entirely unremarkable, and it also means that the cumulative chance over a year is high — which is why one year of trying is the usual threshold for investigation.

That threshold drops to six months at 35, and it is reasonable to seek advice sooner at any age if there is a known reason to.

When to seek help sooner

• Periods that are absent, very irregular, or more than 35 days apart
• Known PCOS, endometriosis, fibroids or thyroid disease
• Previous pelvic surgery, appendicitis with complications, or pelvic infection
• Two or more miscarriages
• Previous chemotherapy or radiotherapy, in either partner
• Known problems with sperm count or quality
• Difficulty with intercourse, or ejaculation problems
• Age over 38, where waiting has a cost of its own

What affects fertility

Age

For women this is the single largest factor, and it acts on egg quality as well as egg number. The decline is gradual through the thirties and steeper after about 37. For men the effect is real but smaller and slower, showing mainly in sperm DNA quality.

Weight

Both a high and a low BMI can disturb ovulation. In women with PCOS in particular, a modest weight reduction is one of the few interventions that reliably restores ovulation in some people. Weight also affects how the ovaries respond to fertility medicines.

Smoking, alcohol and tobacco

Smoking is associated with reduced egg reserve, poorer sperm quality and lower success rates in IVF. Chewing tobacco carries its own risks. Stopping is one of the more effective things within your control.

Medical conditions

Thyroid disorders, diabetes and untreated infections all affect fertility, and all are treatable. This is part of why an initial assessment is worthwhile even when the eventual answer turns out to be simple.

How to read fertility success rates

Success-rate figures are among the most misleading numbers in medicine, and it is worth understanding why before you compare clinics.

A rate can be quoted per cycle started, per egg collection, per embryo transfer, or per patient — and these give very different-looking numbers from identical underlying results. A clinic that only quotes per-transfer figures excludes everyone whose cycle was cancelled.

Rates also depend enormously on who is treated. A centre treating mostly women under 35 will report better figures than one that accepts complex cases and older patients, without being better at anything. And a "pregnancy rate" is not a "live birth rate" — the second is the number that matters and it is always lower.

What is genuinely useful is a frank conversation about the prospects in your specific situation, given your age, your investigation results and your history. That is a figure your specialist can discuss with you honestly, and it is more informative than any headline percentage.