But men and women put different imprints on certain genes, and an embryo with all female imprints fails to develop properly, making men necessary.

For some reason, perhaps the relatively recent origin of the human species, many human embryos are imperfect and fail to develop or implant properly in the wall of the uterus.

The embryos failed to develop and died.

If during previous IVF attempts, the embryos fail to implant in the womb, the doctor may suggest a blastocyst transfer.

But the embryo failed to implant.

The gene was first discovered by biologists who noticed that fruit-fly embryos lacking the gene would fail to form proper segments and assumed a scrunched-up, hedgehog-like appearance.

Current research suggests that fertilized embryos naturally fail to implant some 30% to 70% of the time.

Parasitic twins are known to occur in utero when monozygotic twins start to develop as an embryo, but the embryo fails to completely split.

For example, embryos from homozygous bicoid mothers fail to produce head and thorax structures.

In laboratory experiments, the embryos fail around the 8-cell stage.