"fetus in fetu" is nothing but a hoax.
The Qur'an says it convincingly that Mary had a husband.
6:101 Wonderful Originator of the heavens and earth; how can He have a son when He never chose for Himself a (Sahebah) mate? He creates all things (and does not procreate 112:3.) and He is the Profound Knower of all things and events. [So, how could Mary have a son without a mate? God never changes His Laws operative in the Universe. And, three of the four gospels mention Joseph the Carpenter as the father of Jesus]