Why menstrual cycle does not take place before puberty and during pregnancy ?

Explanation

The reproducting period of a women.s life starts and becomes functional and an increase in sex hormone production, starts only in puberty. So menstrual cycle does not take place before puberty.
The release of a mature egg, maintains the lining of the uterus, during pregnancy. During pregnancy, the placenta, produces progesterone. This maintains the lining of the uterus during pregnancy and it means that menstruation does not happen.