Why the lack of blood takes place during pregnancy?

Dear Student.

I think you wish to ask why menstruation do not occur during pregnancy.

When the sperm fertilizes the ovum and the tiny foetus attaches itself to the uterus, the woman is said to be pregnant. After pregnancy occurs, the brain sends signals to the ovaries to stop the production of the ovum. As ovulation does not take place during this time, there are no chances of having menstrual periods during the nine months of pregnancy. 

Regards,