Animation: fertilisation of egg by sperm

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  • Ovulation (when an egg, or ovum, is released from the ovary) occurs on or around day 14 of a woman’s menstrual cycle.
  • The egg enters the fallopian tube and starts its journey towards the uterus (womb).
  • After intercourse, it takes one to 2 hours for sperm to reach the ends of the fallopian tubes. The sperm usually meet the egg in the outer one-third of the fallopian tube.
  • One of the millions of sperm that have been travelling towards the egg manages to penetrate it.
  • The sperm loses its tail, leaving only the head (which contains the genetic material) inside the egg. Fertilisation occurs when the genetic material from the egg and sperm combine.
  • As the fertilised egg (known as a zygote) continues to travel along the fallopian tube, it starts to divide and form an embryo.
  • When the developing embryo reaches the uterus — which occurs about 3 to 6 days after ovulation — it is made up of about 16 cells, and called a morula.
  • When the morula is a mass of about 100 cells, it forms a structure called a blastocyst, which has a fluid-filled cavity, an outer cell layer and an inner cell mass. The outer layer of cells will form the placenta, and the inner cell mass will form the fetus.
  • The blastocyst usually embeds in the lining of the uterus (the endometrium) within 7 days of ovulation, where it will continue to grow and develop.

Eggs remain viable for about 24 hours after ovulation and sperm usually survive for about 12-48 hours inside the female reproductive tract, so fertilisation is most likely to occur when intercourse takes place in the period from 2 days before ovulation to one day afterwards.

Non-identical (fraternal) twins occur when 2 eggs are released and each egg is fertilised by a different sperm. Identical twins occur when, soon after fertilisation, the dividing cells separate into 2 embryos.


1. Merck Manual of Medical Information. Second Home Edition [website]. Normal pregnancy. Stages of development (updated 2007, Nov). Available at: (accessed 2010, May 11)