Female fertility: a complete guide

The fertility cycle in plain language

Conception requires a chain of events to happen in the right order. Understanding the chain helps make sense of why fertility can be tricky and what affects it.

It starts before you were born

You were born with all the eggs you will ever have. A baby girl has around 1 to 2 million eggs at birth. By puberty this has dropped to 300,000 to 500,000. Roughly 400 of these will mature and be released through your reproductive life. The rest are gradually lost in a process that continues across the decades. This is different from male fertility, where new sperm are produced continuously.

Each cycle is a competition

At the start of each menstrual cycle a small group of eggs starts maturing in the ovaries. Usually one becomes dominant and is released at ovulation. The others get reabsorbed. The released egg travels into the fallopian tube where it has a brief window (12 to 24 hours) when fertilisation can happen. Sperm can survive several days in the female reproductive tract, which is why the fertile window is wider than the egg's lifespan.

Fertilisation and implantation

If a sperm reaches the egg and fertilisation happens, the resulting embryo travels down the fallopian tube to the uterus over several days. Around 5 to 7 days after fertilisation the embryo implants in the lining of the uterus. Implantation is when pregnancy is officially established. Not every fertilised egg implants. Not every implanted embryo continues to develop.

What can go wrong

Each step has its own potential problems. Eggs may not mature properly. Ovulation may not happen. The fallopian tubes may be blocked. Sperm may not reach the egg. The embryo may not implant. Implanted pregnancies may not continue. The mathematics is why even healthy young couples have only around 20 to 25 percent chance of conception per cycle. Most cycles do not produce pregnancy even when everything is working.

When you are actually fertile

The fertile window is shorter than many people realise but wider than the day of ovulation alone. Understanding the timing helps both when trying to conceive and when trying to avoid it.

The fertile window

The fertile window is the few days each cycle when conception can happen. It runs from about 5 days before ovulation to the day of ovulation itself, with the day before and the day of ovulation being the most fertile. The window exists because sperm survive for several days while the egg only survives about a day. Intercourse in this window can produce pregnancy.

How to know when you ovulate

In a regular 28 day cycle ovulation typically happens around day 14, counting from the first day of your period. Cycles longer than 28 days have later ovulation. Cycles shorter have earlier ovulation. Signs of ovulation include changes in cervical mucus (becoming clearer and more stretchy), a slight rise in basal body temperature and sometimes mild pain on one side. Ovulation prediction kits detect the LH surge that happens 12 to 36 hours before ovulation.

How often to have intercourse

Regular intercourse 2 to 3 times a week through your cycle covers the fertile window without requiring precise timing. Couples who track ovulation closely can concentrate intercourse around it but this is not necessary. The obsessive timing approach can backfire by making intercourse stressful. Regular and relaxed beats precise and tense.

Tracking and apps

Period tracking apps can help you spot patterns and predict your fertile window. The predictions are estimates rather than guarantees. Combining app predictions with ovulation prediction kits or temperature charting gives more accurate information. None of this is essential for most couples but can help if you want more information.

The factors that shape your fertility

Several factors affect fertility. Some you can change. Some you cannot. The combined picture determines how easily you conceive.

Age is the biggest factor

Female fertility declines with age, particularly from the mid thirties onward. By age 40 monthly conception rates have dropped to around 5 percent. This is biological reality rather than judgement about timing. Some women conceive easily in their forties. Many others do not. Age does not determine your individual outcome but it shapes the probabilities significantly.

Lifestyle factors

Smoking, very low or very high body weight, heavy alcohol use and severe chronic stress all reduce fertility to varying degrees. The effects compound. Stopping smoking, reaching a healthier weight and reducing alcohol are the lifestyle moves with the biggest impact for most women. The effects are reversible. Improvements show up within months.

Medical conditions

PCOS, endometriosis, thyroid problems, fibroids and previous pelvic infections all can affect fertility. Many are manageable with appropriate treatment. Heavy or painful periods, very irregular cycles or other concerning symptoms warrant assessment to identify any underlying condition. The earlier the diagnosis, the more options you have.

Male factors

Fertility is a couple thing, not just a female thing. Male factor issues account for roughly a third of fertility difficulties, female factor issues another third and combined or unexplained issues the rest. Sperm count, motility and morphology all affect conception. A semen analysis is a routine part of fertility assessment. If conception is taking longer than expected, both partners should be assessed.

Knowing when to involve a GP

Most women conceive within a year of trying. Some take longer. Knowing when to involve a GP helps you not wait too long if there is a problem to identify.

The standard timing

NHS guidance suggests seeking advice after 12 months of regular unprotected intercourse without conception in women under 35 and after 6 months in women 35 and older. These are the standard thresholds for fertility investigation. Earlier consultation is appropriate if there are known concerns.

When to go earlier

Speak to your GP earlier than the standard wait if you have very irregular cycles, heavy or painful periods, known conditions like PCOS or endometriosis, previous pelvic surgery, history of sexually transmitted infections that may have affected fertility or family history of early menopause. The 12 month or 6 month wait assumes nothing else points to a problem.

What investigation looks like

Initial assessment usually includes blood tests (hormones including AMH and FSH, thyroid, sometimes others), a semen analysis for your partner if applicable, an ultrasound and sometimes a test of fallopian tube patency. This identifies most common issues. Specialist fertility services can do more detailed assessment if needed.

Treatment options

Treatment depends on what is found. Many issues have specific effective treatments. Where no specific cause is found, options including ovulation induction and IVF still work for many couples. NHS IVF eligibility varies by area and includes age and other criteria. Private fertility treatment is available with broader criteria but significant cost. The outlook for most fertility difficulties is reasonably good with appropriate treatment.

More on female health

For the dominant fertility factor our How Age Affects Fertility covers age in detail. What Affects Female Fertility covers the broader factors. And How PCOS Affects Fertility covers a common medical contributor.