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Selective reduction: The procedure in which one or more normal foetuses in a multiple pregnancy resulting from assisted conception are destroyed.  The procedure may be hazardous to the remaining foetus(es).

Seminiferous tubules: Very long and convoluted tubules which make up the bulk of the testicles. It is here that sperm is produced.

Sex selection: The sex of an embryo is determined using PGD, in order to avoid sex-linked diseases.

Singleton Birth: The birth of a single baby at a live birth event

Sperm: The gamete (or mature male germ cell) produced by the male, usually through ejaculation. Millions of sperm are present in each ejaculate and roughly half of these will carry X chromosomes, the other half carrying Y chromosomes. A single sperm is called a spermatozoon. 

Sperm sorting: The separation of sperm carrying X chromosomes from those carrying Y chromosomes prior to fertilisation, in order to determine the sex of the offspring. Used for sex selection.

Spermatid: An immature sperm cell. 

Stem cell: Undifferentiated primitive cells whihc have the capacity to develop (differentiate) into a large number of different cell types.

Stillbirth: The birth of a dead infant. 

Stimulated cycle: A treatment cycle in which stimulation drugs are used to produce more eggs than usual in the woman's monthly cycle. 

Stimulation drugs: Drugs used to stimulate a woman's ovaries to produce more eggs than usual in a monthly cycle; also known as superovulatory drugs. 

Subzonal sperm insertion (SUZI): A technique whereby one or several sperm are injected directly through the zona pellucida (outer layer) of the oocyte.

Superovulation/stimulation: The medical stimulation of the ovary with hormones to induce the production of multiple egg-containing follicles in a single menstrual cycle.

Surrogacy: The process of a woman carrying a baby for another person. Straight/full/traditional surrogacy involves using the egg of the surrogate mother and the sperm of the intened father. Host/part/gestational surrogacy uses the egg of the intended mother and the sperm of the intended father or donor sperm.

Swim up: A technique for separating sperm, based on their ability to swim through a liquid.