A US woman and her husband are suing a fertility clinic they say implanted a stranger's baby, lawyers said Monday.

When Daphna and Alexander Cardinale first saw their newborn, conceived through IVF, they noticed the baby girl had jet-black hair and a much darker complexion than anyone in their family.

A DNA test several months later revealed the girl born in September 2019 was not related to either of them, and they had been raising another couple's child.

Lawyers in Los Angeles say two laboratories connected by one doctor switched the embryos of two entirely separate couples and implanted them in the wrong mothers.

The couples eventually met and worked through the courts to gain custody of their genetic children.

"The moment our... daughter was born should have been among the happiest of my life," Alexander Cardinale said.

"But I immediately felt shaken and confused as to why I didn't recognize her.

"When the truth came to light, it made exchanging the children all the more heartbreaking," he said.

"Losing the birth child you know, for the genetic child that you don't know yet... a truly impossible nightmare."

The suit is seeking to recover emotional damages, compensatory and property damages and a wide range of costs, lawyers Peiffer Wolf Carr Kane & Conway said.

In IVF, the sperm and egg are combined and cultivated in a laboratory setting before the resulting embryo is implanted into the mother's uterus.

The procedure is often used when a couple has trouble conceiving. It can also involve the donation of either eggs, sperm or both.