More properly, in modern French, en ventre de sa mère; in the belly of its mother.
When wills state that an estate shall vest to all grandchild living or alive at the time of the death of the testator, that estate has been held to vest in a share to any fetus alive in the belly of the testator's daughter, daughter-in-law or sister-in-law as the will may provide.
To recognize this unique entitlement, common law developed the concept of en ventre sa mère. This, contrary to the standard approach of the law towards the fetus; that a child has no legal standing until and unless born alive.
In Doe v Clarke, the deceased left a sizable English estate to his brother unless his brother died before him, in which case the lucrative estate was to go:
"... to the use and behoof of all and every such child or children whether male or female of said brother as should be living at the time of his decease."
The brother died shortly after the testator, leaving behind a pregnant wife, who delivered the testator's niece seven months later. The use and behoof entitlement of the newborn was at issue, but quickly dealt with by Justice Eyre:
"It is plain on the words of the will that the testator meant that all the children whom his brother should leave behind him should be benefited. But independent of this intention, I hold that an infant en ventre sa mère who by the course and order of nature is then living, comes clearly within the description of 'children living at the time of his decease'."
- Burrows v Cleghorn  2 Ch 497
- Doe v Clarke 126 ER 617 (1795)