Last month's Accountancy (p31) reported that only one partner in 11 in top accountancy firms is female. This may or may not be due to overt discrimination, but nevertheless increasing numbers of professional women are choosing to delay childbearing until they've made it through the career bottleneck.

The question many of my professional patients ask me is, 'How late can I leave it while still being assured of a normal pregnancy and a healthy baby?'

The short answer is that no woman of any age is assured either of these things (5% of women seeking pregnancy will never conceive) - but it's certainly true that from a purely physiological point of view, the best age to start your family is around 17 and that your chances of producing a baby naturally drop dramatically from around age 35. The 'Bridget Jones syndrome' (needing to find a man and settle down once you turn 30) has a sound medical basis, and it has major knock-on social implications since the career ladder gets especially tough around that time.

Conceptual advice

So how can you swing the odds in your favour if you choose to delay motherhood?

You can't change your age, so what can you change? If you are markedly overweight (a state known to lower fertility), losing weight steadily to achieve a trim (but not skinny) body size can almost triple your chances of successful pregnancy. And if you are very underweight (a body mass index - your weight in kilograms divided by the square of your height in metres - below 18.5), gaining just 3kg also triples your odds.

Giving up smoking increases your fertility by between 20% and 50%. Reducing a high alcohol intake to fewer than one drink a day increases it by about the same amount. And reducing your caffeine intake from more than five to less than one cup of coffee a day improves your odds by a further 25%.

On purely statistical grounds, you could also try trading in your man for a younger model. If two women of identical age had partners aged 30 and 50 respectively, the former would have a 30% greater chance of achieving pregnancy within a year. Curiously, the sperm counts wouldn't be that different - and medical experts have coyly speculated that it must be the frequency of lovemaking or even the positions adopted that explains the difference. On second thoughts, changing partner might make you vulnerable to a sexually transmitted disease, which could endanger your ageing tubes, so better to stick with the devil you know.

Test tube lottery

What about the various forms of assisted reproduction - the true 'test tube' conception or in vitro fertilisation (IVF), and various halfway houses such as gamete intrafallopian transfer (GIFT)? We all know of someone who has successfully produced triplets after years of infertility, but most of us also know people who have spent thousands on repeated cycles to no avail. The women with the best chance of success are those who are otherwise healthy, with regular, unproblematic periods, who seek treatment before the age of 35, have a 'tubal' cause of infertility (such as Chlamydia or a past ectopic pregnancy), and have not had previous failed attempts at IVF. Those least likely to ever achieve a pregnancy are those over 45, with 'unexplained' infertility, previous recurrent miscarriages or failed IVF attempts, chronic health problems such as diabetes or kidney trouble, and very irregular periods. Younger women (below 35) have an equal chance of a successful IVF pregnancy whether they use their own eggs or donor eggs, but a 40-year old has three times, and a 45-year old 8 times, the chance of a live birth if she uses donor eggs.

If you've read this far, you probably want children at some stage, so my advice is go for it sooner rather than later and the devil take your career, since your ovaries will age more quickly than your brain. I've never had a mother weeping in my surgery saying she wished she'd chosen an executive career instead, but I've had plenty of childless professional women with regrets. A typical story is 'I thought I was safe delaying pregnancy because I had an abortion at 19'. But the fact that you conceived without problems 20 years ago doesn't mean you'll carry a healthy pregnancy in middle age.

And what if you're male? Frankly, I'd be highly suspicious if your thirtysomething junior partner invites you round to her place for dinner after reading this article.