In vitro pregnancy starts only when fertilised eggs are implanted in a woman's uterus, and not at the moment they are fertilised with sperm outside the body, the European Union's highest court ruled on Tuesday.

The court was reviewing the case of Austrian waitress Sabine Mayr, who demanded compensation from her employer, a Salzburg restaurant, for being fired while undergoing in vitro treatment.

On the day Mayr received the dismissal letter, her eggs had been fertilised with her partner's sperm but not yet transferred to her uterus. The European Court of Justice said that was not enough to protect her under Austrian maternity law.

"The protection against dismissal ... cannot be extended to a pregnant worker where, on the date she is given notice of her dismissal, the in vitro fertilised ova have not yet been transferred into her uterus," the court said in a statement.

This is because the transfer of the fertilised eggs to the uterus could be postponed for years or even abandoned, the court said. Fertilised ova can be frozen and stored..

"However, a worker who is undergoing in vitro fertilisation treatment can rely on the protection against discrimination on grounds of sex," the court said.

Because men cannot undergo in vitro fertilisation, the ECJ said that if the Baeckerei und Konditorei Gerhard Floeckner restaurant had fired Mayr because of her treatment, that would constitute direct discrimination on the grounds of sex.

The ECJ asked the Austrian courts to establish whether Mayr had been fired because of the treatment.