Antenatal screening has become a routine part of pregnancy. But Joanna Moorhead, pregnant with her fourth child, has refused all tests and believes more women should do likewise

For generations, the standard question you got as a pregnant woman was "when's it due?". But no longer. "Do you know whether it's a boy or a girl?" is what I'm most frequently asked when someone spies my bump. When I say I've no way of knowing because I'm not having any antenatal tests – no, not even ultrasound scans – people are generally flabbergasted. Isn't ultrasound a routine part of antenatal care in this day and age? Doesn't everyone have to have a scan?

The answer is no, and no again. Antenatal screening is now universally offered in the UK, in one form or another, and it's true that the vast majority of pregnant women take up the offer of at least one scan. But it certainly isn't – or shouldn't be presented as – something you've no choice about. Ultrasound scanning, although it might be terribly moving and hugely involving for the parents-to-be, certainly isn't about giving you a pretty take-home picture for the family album, still less to help with the bonding process. The hard truth about ultrasound tests is that they're there to screen or diagnose problems or potential problems in the baby-to-be. Which is why if, like me, you'd rather not know if your baby has anything wrong with it until the day of the birth, you're best off declining politely when you're offered a scan or a blood test.

The trouble is that more and more pregnant women and their partners seem to overlook the strings attached to screening tests in general, and ultrasound scans in particular. The flip side of that is that they can be horrified when they're told there is, or might be, a problem. Women such as Sue Carter (not her real name) who with her partner James was looking forward to her "routine" antenatal scan and was shocked to be told it showed she had an "above average" risk of the baby having Down's syndrome. The scan she had, at 11 weeks, was in fact a special screening scan called a nuchal fold scan in which a "marker" for Down's involving a fluid sac at the back of the embryo's neck is looked at in detail. The trouble is that Sue hadn't taken in what the scan was actually checking for – she just thought it was something everyone went along to, so went along too.

Sue hadn't given the consequences of her scan a second thought – suddenly, she was thrown into the turmoil of having to decide whether to live with the question mark that had been placed over her baby's condition, or to opt for a conclusive test – an amniocentesis – which carried a small risk of miscarriage. "I decided I couldn't live with the uncertainty in the end, and we went ahead with the amnio," she says. "The results took two weeks to come through, and it was hell. By that stage I was just starting to feel the baby move, and to be honest I thought I'd never be able to go through with a termination."

The news was good – the baby hadn't got Down's. But Sue resents the fact that her pregnancy was put "on hold" for several weeks, and that she had to live with the uncertainty. In another pregnancy, she says, she'll think very carefully about whether to have a nuchal fold scan or not.

Unfortunately, though, it's usually only women like Sue who've been on the sharp end of antenatal testing who give it a lot of thought before they turn up at the hospital on the appointed day. In fact in at least one area of the UK the nuchal fold scan is now a pregnant woman's very first contact with the maternity services – antenatal care kicks off with a letter suggesting a date and time for the scan, after which the woman gets her first appointment with a midwife. With the letter comes a brief outline of what the scan involves, but clearly it's being presented as routine – and clearly if you turn the scan down, you'll be "stepping out of line" with what's considered "normal".

Linda Thompson of the group Antenatal Results and Choices, which offers telephone support and counselling to women embroiled in the testing process, agrees that in many areas not enough information is being provided ahead of the very first screening. "We get a feeling that while screening tests are a choice that can be made, many women find themselves being caught up in this path. There's a real need for a lot more input before, during and after the process so that women really understand not just what the tests they're being offered are for, but what the information they've given really means."

Typically, screening tests – usually either the nuchal fold or a blood serum screening, which checks a blood sample for the presence of chemical indicators – are considered by clinics to be "positive", ie "high risk", if they suggest a foetus has a higher than one-in-250 risk of having a particular condition, usually Down's. Most women get completely freaked out to be told they've tested "screen positive" – but put another way, they're actually only being told they have a less than 0.5 per cent chance of having an affected baby. "How the information is presented, and how much a woman has already been prepared as to what these statistics mean before she's given them, matter a huge amount," says Ms Thompson.

I'm lucky in that I'm being looked after in my pregnancy by a team of independent midwives who are less orthodox on issues like antenatal testing than many midwives within the NHS, so no one has tried to change my mind and persuade me into a scan. In my last pregnancy, though, I made the mistake of telling an obstetric registrar that I'd turned down my nuchal fold scan, and he was amazed. "Why?" he asked, genuinely astonished. Because, I said, I wouldn't want a termination – so why bother going through with the tests, particularly as the further tests, such as amniocentesis and CVS (chorionic villus sampling), are invasive and so carry a risk of miscarriage? He didn't have an answer, but he was clearly surprised.

Here's my logic – though if you're an obstetrician, I don't expect for a minute you'll understand. The fact is that the vast majority of pregnant women go on to have entirely healthy babies – a fact I think the screening process sometimes encourages us to lose sight of. Even at my relatively advanced age of 38, I have a better than 99 per cent chance of having a baby unaffected by Down's or any other syndrome.

But there's more to it than that. There are all sorts of things that can go wrong with a human being, both before and after birth. It seems to me that taking on a baby is taking on that risk – the risk that you'll find yourself caring for, coping with and suffering on behalf of a child you've brought into the world. Antenatal screening and testing can check for a very few certain specific conditions, but it can't – and never will be able to – check for absolutely everything. One of the problems with the antenatal testing process is that it gives pregnant women the impression that they're "out of the woods", when in fact no one can ever promise you anything of the sort.

OK, so maybe you find your child hasn't got Down's syndrome – but he could have cerebral palsy, he could be deaf, he could have some mysterious syndrome that isn't detectable in the womb. Even some conditions that are, in theory, detectable, aren't always noticed – one friend of mine had a baby with a potentially lethal heart condition in which the main arteries are transposed. She'd been given a clean bill of health at her scan, despite the fact that in the hands of a skilled operator, this defect can be picked up.

I don't want to impose my beliefs – strong though they are – on others. I completely understand the position of one friend who took completely the opposite line to my own in her recent pregnancy. She had a nuchal fold scan, which gave her what was considered a low risk of Down's – but she couldn't rest easy. She knew in her heart that she couldn't live with bringing a child with Down's into the world, so she decided to go ahead with an amniocentesis. We stand at opposite ends of the spectrum, but she respects my position and I respect hers. Both viewpoints are, in the buzzwords you often hear in antenatal clinics, "informed choices". We each knew what we were doing in accepting or declining a test, and why. The trouble is that we may be exceptions, because the evidence suggests that many women getting sucked into the antenatal screening process are making what's very far from an "informed choice" in the first place.

The Antenatal Results and Choices helpline is on 020-7631 0285