Blood tests may soon be able to inform pregnant mothers whether they are carrying a boy or a girl less than three weeks into their pregnancy.

New research has found that it is possible to tell almost from the start of pregnancy whether the foetus is male or female, based on the levels of a hormone called called human chorionic gonadotrophin (HCG) in the mother's blood.

"We've discovered that levels were increased as early as 16 days after fertilisation," said Dr Yuval Yaron, the director of the prenatal genetic diagnosis unit at the Tel Aviv Medical Centre in Israel. Boys and girls produced "statistically significant" differences in the levels of HCG in the blood, with female embryos leading to levels 18.5 per cent higher, on average. The presence of HCG is already used to detect pregnancy in home-testing kits.

Hundreds of would-be parents spend thousands of pounds every year on expensive "sex selection" techniques to implant embryos of a specific gender. None has a guaranteed result, but there is significant pressure for the sex of babies in the womb to be detected earlier. Currently, it can be done four months into the pregnancy.

But Dr Yaron said his finding could not yet be used to determine the gender of a baby because too few women have HCG levels high enough to make a reliable prediction. "It would be possible to predict the sex of a foetus if we can identify other markers that also demonstrate early gender-related differences. We are working on this now, and hope to have some results soon."

He suggested that being able to identify the gender of a baby would be useful for parents who had severe inherited sex-linked conditions and might want to know early on whether to continue the pregnancy.

But Peter Garrett, the research director of the anti-abortion charity Life, said. "Although the technology is neutral, the evidence from around the world suggests people will use this to enable them to choose surgical or pharmaceutical abortions, based on the baby's sex. Selective abortion is morally reprehensible. Even if they have a serious sex-linked illness, the baby has a right to life, and that starts when it begins its existence."

At the start of pregnancy, HCG helps the running of the corpus luteum, a temporary structure formed in the ovary after the egg is shed, which produces the hormones oestrogen and progesterone until the placenta takes it over.

Dr Yaron said the differences in levels of HCG must be caused by the way the placenta reacts to female pregnancies, rather than to any hormones produced by the embryo, because at such an early stage there are no glands in the embryo capable of producing those chemicals.

The team is now searching for genes that might control hormone output in the placenta, which have been mapped to the X chromosome.