A woman who had fertility treatment at a clinic in Rome became pregnant with the twins of another couple after their embryos were mixed up.
Italy's Health Ministry is launching an investigation into the error, which was discovered only when the woman was three months pregnant.
The mistake happened on December 4, when four different couples were receiving treatment at a specialist fertility unit at the Sandro Pertini Hospital in Rome, La Stampa reported.
It is not clear if the mistake led to any of the other women becoming pregnant with the wrong baby.
Health Minister Beatrice Lorenzin said investigators would look at whether the hospital had "respected all the legal procedures" and expressed concern she had only learned about the mistake through press reports.
"The national standards on assisted fertilisation, which are based on European directives, are very rigorous, and if applied correctly, guarantee the traceability of all biological material used in the reproduction process," she said.
The Rome health authority said it became aware there was a problem only on March 27. It has halted all embryo implantations at the clinic until further notice.