Weight is the fertility factor you can change the most.

Too much or too little body fat changes the hormones that control ovulation in women and sperm production in men.

Extra fat raises insulin and oestrogen levels, which can stop regular ovulation, and it often comes with PCOS. Being underweight, or dieting and exercising very hard, can switch ovulation off altogether. BMI (weight compared with height) is the usual screening measure, with categories from underweight to healthy, overweight and obese, but waist size matters too.

Yes, excess weight in men is linked to lower sperm count, movement and DNA quality, and in women to a weaker response to IVF medicines.

In IVF, women carrying extra weight often need higher doses and longer stimulation, and scans and egg collection are technically harder. In pregnancy, the risks of miscarriage, gestational diabetes, high blood pressure and caesarean birth rise. That is why weight is part of the plan for both partners, not a side remark.

Tests look at weight, height and waist, and at the conditions that travel with weight: blood sugar, thyroid, blood pressure, cholesterol and vitamin D.

For her, ovulation is tracked and PCOS is looked for. For him, a semen analysis. These results decide whether weight is the main issue, one issue among several, or not the issue at all.

Losing some weight helps most when periods are irregular or you are not ovulating, but whether to wait, and for how long, depends on your age and egg reserve.

In women who do not ovulate, a modest weight loss can restart ovulation without any medicine. For older women or those with low egg reserve, months of waiting can cost more than weight loss gains, so treatment and lifestyle change go side by side. Your target, and the time to reach it, is set with your doctor and a dietitian [CONFIRM: in-house or referral], not taken from a chart or a diet trend. Asked which factor couples can influence most, Dr Vani starts with weight.

Start with steady changes you can keep for months, together: regular meals, more movement every day, better sleep, no smoking and less alcohol.

Crash diets, very low calorie plans and some fashionable diets can leave you short of iron, folic acid, vitamin B12 and iodine, which matter before and during pregnancy. Start folic acid now if you are trying. If you have lost a lot of weight quickly or had weight-loss surgery, tell us before trying, because pregnancy is usually delayed for a time after surgery.