As any pregnant woman will tell you, digestive problems are just one of the many joys of impending motherhood. Heartburn, acid reflux, indigestion - you name it, I've had it.

Frustratingly, though, there's little my GP can do about it, because most conventional medicines are off limits until well after the baby is born. And with good reason, says toxicologist Rachel Spencer. "When drug companies perform clinical trials, it's unlikely that they'll test any medicines on pregnant women because of the potential risk to the foetus. Instead, they leave it up to your GP to decide whether it's safe for you to take a certain medicine - and most doctors, unsurprisingly, don't want that legal responsibility, either."

But what about herbal remedies? I had taken aloe vera juice before I was pregnant and found it very helpful for tummy problems. My local healthfood shop checked the bottle and couldn't find any contraindications for pregnant women. Surely it would say if it wasn't safe to take?

Apparently not. Luckily, I checked - it turns out that aloe vera products may trigger uterine contractions and should be avoided during pregnancy. Kaz Cooke, author of the Rough Guide To Pregnancy, warns, "Some preparations can be taken at different times during pregnancy; others are dangerous at any stage."

Consider ginkgo biloba, for example, a common herbal remedy often taken to alleviate poor memory. Recent research at Wayne State University in Detroit found that pregnant women who used ginkgo biloba had high levels of colchicine, a naturally occurring toxin that has the potential to cause birth defects.

Even herbal medicines that are recommended for pregnant women are not all straightforward. Raspberry leaf tea, taken during the last eight weeks of pregnancy, prepares the uterine muscles for labour, making childbirth easier. However, its very effectiveness at encouraging uterine contractions makes it a complete no-no in the early stages of pregnancy, because it could increase risk of miscarriage.

Herbs to avoid during pregnancy include such common remedies as goldenseal and St John's Wort (both thought to trigger uterine contractions), feverfew (which can induce bleeding), and ginseng (linked to gestational diabetes; for a list of herbs to avoid when pregnant, see Herbs For A Healthy Pregnancy, by Penelope Ody, published by Keats).

So what's a girl to do if she's pregnant and feeling under par? "To use over-the-counter herbals during pregnancy is crazy," says Dr Ann Walker, senior lecturer in human nutrition at the University of Reading and a registered medical herbalist. "The rule is that women should never take anything during pregnancy unless it is absolutely necessary."

"It's important to seek advice from a professional if you are pregnant," says medical herbalist Elizabeth Harrison. "Members of the National Institute of Medical Herbalists undergo extensive clinical training and are committed to the safe application of herbal medicines."

In other words, self-prescribing is just not a good idea.

· National Institute of Medical Herbalists, 01392 426022.