Can you tell how fast your body clock ticking?

New studies into female fertility have made several startling findings.

Not only may fertility levels start to drop off after the age of 30 (rather than 35 as previously thought), but now overexercise, dieting, stress, eating unhealthily and high consumption of alcohol and nicotine have also been found to have a devastating effect.

Anaemia and sexually-transmitted infections, such as chlamydia, can affect fertility, while conditions, such as fibroids and endometriosis, can make conception impossible. Little wonder that one in six couples in the UK experiences difficulty conceiving.

So just how much longer do you have left, before your age, diet, lifestyle and bad habits will make it impossible for you to have a baby?

We asked Dr Gill Lockwood, fertility expert and medical director of the Midland Fertility Service, near Walsall, to tell us how long it will be before these five very different women could become infertile.

Joanna Witt, 28, is a journalist, from North London. She is single, has a BMI of 21.6, and says:

Through my job, I'm invited

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I try to get seven hours' sleep a night, but sometimes it's only about five. I eat a balanced diet with lots of fruit and wholegrain foods and I cook most nights. I also exercise once a week and have regular periods.

I used to smoke about ten cigarettes a night, but stopped a year ago.

DR LOCKWOOD SAYS: A Danish study claims abstinence from alcohol makes you more fertile. However, I've found some babies actually result from couples relaxing over a bottle of wine, so I believe moderation is called for.

Joanna will have to tone down her lifestyle when she is pregnant or she risks harming the baby. She should also try to exercise at least three times a week, to prepare the body for the rigours of pregnancy.

The occasional cigarette doesn't affect fertility, but any woman who smokes regularly will have damaged her fertility. Those who have smoked between ten and 20 cigarettes a day for more than ten years will advance the age of their menopause by up to four or five years.

Heavy lifetime smoking affects the quality of the eggs and sperm - and the ability of it to be fertilised. In IVF treatment, smokers have a reduced pregnancy rate of up to 40 per cent.

HOW MUCH TIME LEFT? Seven years - a little champagne may even aid conception, though she'll need to change her lifestyle once she's trying for a baby.

JOANNA'S RESPONSE: I'm relieved my smoking hasn't affected my fertility, but seven years isn't that long.

This has motivated me to think about increasing my exercise and look at my lifestyle long-term because I do want children one day.

Anne Muckley, 40, is single and has no children. She owns a complementary therapy centre and lives in Docklands, London. She has a BMI of 23.4 and says:

I would like children but,

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I know there's been debate about older women leaving it too long, but I know my own body. My menstrual cycle is regular and I feel healthier than ever.

In my 20s and 30s I drank and smoked a lot, but I stopped a few years ago, and now I eat plenty of vegetables and fish.

I also do Pilates and T'ai chi, and I keep relaxed with complementary therapies such as massage.

DR LOCKWOOD SAYS: I'm afraid the chances of conceiving for any woman over 40 are grim, particularly for a woman, like Anne, who has never had a baby.

At least women who have conceived before, even if they haven't carried it full term, have shown there are no major problems with their reproductive organs.

It doesn't matter how young she looks on the outside, what skincare regime she follows or how often she goes to the gym - inside, Anne's eggs are still 40 years old.

By the time a woman is 40, she has between a two and five per cent chance of conceiving naturally each month, compared with the 25 per cent chance of a woman in her 20s.

Even if she does conceive, she has a higher risk of miscarriage or having a baby with Down's syndrome, because the quality of her eggs will have deteriorated with age.

According to the data available, 40 per cent of all pregnancies achieved by women aged 40 or over, whether spontaneously or through IVF, will unfortunately end as a miscarriage.

HOW MUCH TIME LEFT? Less than a year. Even if she started trying for a baby now with a healthy, fertile partner, her chances of a successful pregnancy would be low.

ANNE'S REACTION: I'm not worried because the statistics don't take into account the wide spectrum of individuality. My mother had two children in her 30s and I genuinely believe it's down to genetics - I feel instinctively I have three years left.

Michaela Baptiste, 17, is a dancer, who lives in East London. She has a BMI of 19.1 and says:

I dance every day for about

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I have a light breakfast, then fruit snacks during the day, followed by a light evening meal, for example, salad, eggs and potatoes at around 6pm.

I don't eat in the evenings because I don't think there's enough time to digest your food before you go to sleep.

I have a regular menstrual cycle.

DR LOCKWOOD SAYS: Even though Michaela says she doesn't starve herself, her BMI is low and I can't see how she can get enough calories to sustain hours of dancing from such light meals.

She also has to be careful of an iron deficiency if her diet lacks class-one protein, such as red meat.

Strict vegetarians, for instance, can be anaemic, and anaemics can have fertility problems because iron is vital for making blood cells.

To maintain a healthy body weight, Michaela, like other athletes, would need to eat more than the average person as she is burning off so many calories. I'd say her diet and activity level is setting her up for fertility trouble in later life, even though she has regular periods.

I've seen people with a low BMI, such as Michaela, who still have periods then discover they can't conceive.

Low levels of body fat prevent sufficient oestrogen being made, which not only affects fertility but also increases the risk of osteoporosis. Cases of miscarriage and preeclampsia are higher in thin women.

However, age is on Michaela's side, so she still has time to put on weight if she wants to conceive.

HOW MUCH TIME LEFT? Fifteen years, but her low BMI and extremely active lifestyle could be storing up problems for the future. She needs to eat more and exercise less to maximise her fertility.

MICHAELA'S REACTION: I am still very young and dancing is my priority at the moment. I know I'll put on weight when I get older, and I definitely want to have children.

Caroline Mirams, 32, an executive assistant to a firm of headhunters, is married to Phil, 33, and comes from Eastbourne. She has a BMI of 20.2 and says:

I plan to start trying for

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I stopped using normal deodorant because I read it contains a chemical that can affect your baby's development. I have regular periods and eat a balanced diet. DR LOCKWOOD SAYS: There is no need for Caroline to go overboard with her diet, as long as she eats healthy and balanced meals.

In fact, although it may appear she is doing everything possible to boost her fertility, being too anxious about it can have a negative impact. Stress produces the hormone prolactin, which suppresses ovulation.

Taking folic acid is important, as the evidence that it can help prevent spina bifida is overwhelming. However, more extreme precautions such as stopping using deodorants, isn't necessary.

Three months before trying for a baby, a woman should start reducing alcohol, getting fitter, cutting out medication and stop smoking.

There are people who read up on everything about childbirth, but nothing is guaranteed because pregnancy is a lottery - you can do everything right and still run into problems orit can hap --pen by accident and go verysmoothly .

It's good that Caroline is planning to start trying for a baby soon, because at 32 her fertility will start to decline.

HOW MUCH TIME LEFT? Four years. But she's too anxious, stressed and thin for an easy conception. She needs to worry less and eat more.

CAROLINE'S REACTION: I feel relaxed after hearing what the doctor has said, although she's made me realise I can't leave it too long to have children. They will have to bemore of a priority now.

Sarah Tingey, 30, an architect, is single and lives in Chelsea, London. She has a BMI of 22.6 and says:

I've always wanted to reach a certain level in my career before thinking

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I lead a busy life - I exercise in the mornings four times a weekand work long hours. I also socialise about four times a week and drink three or four glasses of wine. I don't smoke, and eat a low-carbohydrate, high-protein diet. I have a regular menstrual cycle.

DR LOCKWOOD SAYS: At 30, Sarah should find she is able to conceive relatively easily, though her fertility will already be declining. But while it's understandable to attain a certain level of stability, she shouldn't leave it much longer.

There's nothing harder than seeing a woman in her mid to late-30s who has worked hard for success, but then finds she can't have a baby.

The quality of eggs and sperm gradually deteriorates with age, and while you may feel young and fit at 35, your eggs might not. A generation ago, hardly any women over 35 had babies - now 12 per cent of pregnancies in the UK are born to women of 35or older .

My message to young career women, who know being a mother will be important to them one day, is not to delay it.

HOW MUCH TIME LEFT? Five years. She says she's too busy at the moment but she needs to start thinking about it .

SARAH'S REACTION: I'm surprised she's advising me to start thinking about babies so soon. I've always been aiming for 35.

I've got friends in their mid-30s who've had babies without problems, so I think it's very much an individual thing.

Midland Fertility Service: 01922 455 911 or visit www.midlandfertility.com