Is it a girl or a boy? It's the first question every new mother asks. Except for those, like me, for whom the moment of birth seems much too far away to discover such crucial information. I'm not even at the halfway point of my pregnancy, yet I'm champing at the bit to find out and have already had a stab at crystal dowsing and an online Chinese gender predictor. Alas, the results contradicted each other, but thanks to the wonders of ultrasound, a baby's sex can usually be seen at the 20-week scan - which for me is just 10 days away.

The problem is that pregnant women - unfamiliar as most of us are with reading scans - are dependent on the scan operator to tell us, who in turn are dependent on their hospital's policy of informing parents-to-be. Many get away with a blanket ban because, although guidance allows hospitals to tell the child's gender, they are not required to do so.

At my first maternity-related visit to Watford General Hospital, I was firmly told to put aside any inquisitiveness I might have. "The purpose of an ultrasound scan is to date a foetus and check for abnormality. It is not for telling people the sex of their child," a spokeswoman from West Hertfordshire Hospitals NHS Trust has since stated.

But unless the baby is in an awkward position, scan operators will tell you that they can easily tell the gender at 20 weeks. The fact that the only thing stopping them from passing this information on is the powers-that-be waving a finger of warning at them smacks to me of outdated, patronising and authoritarian medical values. "It's my body and my baby," my now heavily pregnant best friend - also attending Watford General Hospital - recently lamented. "Who do they think they are by withholding something like that? It's not as if it costs them anything to tell."

Adding insult to injury is the fact that it did wind up costing her. She - and at least two other women I know - were told that they were welcome to book in to see the same scan operator in his private practice for a fee of £100, where he would be happy to tell them whether to paint the nursery pink or blue.

Other hospitals - including those in the North West London Hospitals NHS Trust - say they won't inform parents of the sex of their baby "largely because we cannot be 100 per cent sure of the sex and we would not want to cause disappointment to families".

My guess is they don't want to be sued. Several women I've spoken to have been told as much by scan operators off the record. Surely a system can be brought in whereby the parents sign a disclaimer or the scan operator can be - as indeed most already are - clear in saying that they can never be absolutely certain.

While no hospital that I spoke to for this article was willing to enter into a discussion about it, there is also the cultural issue. Indeed, many women say they've heard that their local hospital's blanket ban exists because some sections of ethnic minorities are interested only in having boys and so those who are told they've having girls abort. "I worked for a long time in Leicester, where this myth was prevalent," says Sandra Wheatley, a social psychologist with a special interest in parenting. " But it's dangerous for hospitals or anyone else to invest anything in this myth because there is absolutely no evidence for it and it has the result of stereotyping people because of the colour of their skin."

North West London Hospitals NHS Trust states that another reason for its ban on disclosing the sex of the baby at the 20-week scan is that "there is no clinical benefit". Granted, it's hard to argue against that, but there are definite psychological benefits to finding out. I've thought long and hard about why I want to know so badly, and I think part of it is that it will make my baby seem "real". For most of my adult life, I've been told that it would be virtually impossible for me to conceive and so even though my husband and I have - for the two months that we've known I'm pregnant - been in a state of elation, we still can't quite get our heads round it.

Wheatley confirms this attitude isn't uncommon. "A lot of women say to me - particularly the ones who have an unplanned pregnancy or who have had difficulty conceiving - that they find it very difficult to relate to the fact that this really is a baby, even at the 20-week scan, where it very clearly looks like a baby," she says.

"These women find it much easier to relate to that baby if they know it's a boy or a girl. A baby is a baby, but a boy is a son and a girl is a daughter. That's something they can relate to because they're a son or a daughter. I've found this is often the moment where women really feel like a mother for the first time. Psychologically speaking, I'd say it can therefore be very important for women to know what they're having. In fact, I'd go so far as to say that it can be virtually critical in helping women cope with the massive psychological transformation that motherhood inevitably brings."

She adds that for many women, knowing the gender of their baby can also help with the bonding process, particularly if there is a preference for one or other sex. "If someone particularly wants a girl because, say, they already have two boys, then I think it gives them a chance to adjust to the fact that it's not what they'd hoped for before that child comes into the world. It's not as if people expect to have one particular gender - people realise the odds are 50/50 and you get what you've given - but it allows them to get used to the idea earlier on."

Of course, there are some parents-to-be for whom their thirst for knowledge about the gender amounts to nothing more than what colour scheme to go for and what names to consider. But just because they're not earth-shattering issues doesn't mean they should be belittled.

As luck would have it, my recent house move means that I'm now under Heatherwood and Wexham Park Hospitals NHS Trust, whose ultrasound unit's attitude to disclosing the gender is both adult and refreshing. But it's not good enough that women who want to know are reliant on a postcode lottery. It's high time that our NHS system - and particularly those trusts with bans - grow up and wake up to the rights and needs of pregnant women.