This thickening of the uterine wall is not permanent. If the egg which is the female germ cell gets fertilised, it develops into a single celled zygote and starts multiplying to become an embryo which travels down the fallopian tube and gets embedded in the uterine wall which results in pregnancy. During pregnancy no more eggs are released, and the thickened lining is discharged only after the gestation period of 36 weeks and the baby is expelled from the womb. However, if fertilisation does not occur, the released egg and the thickened lining are shed off resulting in menstruation.