Animation: fertilisation of egg by sperm
Take a look at our animation to see what happens during the process of fertilisation. Use the ‘Next’ navigation button to move through the scenes. You can also use the ‘Back’ navigation button to review scenes.
- 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.