Pronuclei (PN) and normal or abnormal fertilisation

Pronuclei (PN) are small round structures containing genetic material that briefly become visible inside a fertilising egg, one from the egg and one from the sperm. Seeing exactly two pronuclei (2PN) at the fertilisation check is the usual sign of normal fertilisation. Seeing none (0PN), one (1PN) or three or more (3PN) pronuclei suggests fertilisation has not proceeded in the expected way, and such eggs are not usually selected for transfer, though laboratory practice around borderline findings varies by clinic. These observations describe what the embryology laboratory saw at a single early time point; they are a quality-control step in the laboratory process, not a judgement on a person's fertility or on any embryo that later forms from a normally fertilising egg.

Also known as: PN, 0PN, 1PN, 2PN, 3PN, normal fertilisation, abnormal fertilisation

Last reviewed: 9 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.