Plural of testis. One of the two male reproductive glands located in the scrotum. The testes produce sperm and male sex hormones.

Twin-to-twin transfusion syndrome

Syndrome caused by the fact that fetuses share the placenta unequally, which leads to imbalanced circulation between the placenta and the fetuses.

Two-pronuclear stage

An egg in the process of fertilisation, before the nuclei of the egg and sperm (pronuclei) have fused.