What happens?

The doctor will carry out a scan to find the fetus and its position. Throughout the procedure, which usually takes no longer than 30 minutes, your doctor will watch the baby's movements and position on a screen. 

As soon as the doctor is sure of the fetus' position in the womb, he inserts a fine needly through the abdominal wall into the uterus and removes 15-20ml of amniotic fluid. It isn't done under anaesthetic, because it's not painful, though it might be uncomfortable - but no more so than a blood test.

It's a very quick procedure, though in rare cases (if the placenta is at the front of the uterus or if you have a negative Rh blood group) it can take longer. If you have a negative Rh blood group, after the sample is taken you can have an anti-D immunoglobulin injection to prevent you from developing antibodies against the baby's blood cells.