Pregnancy and cessation of menstraution

In the Name of Allah, The Most Gracious, Most Merciful
If it is a well-known fact that there can be no pregnancy with menstruation, nor can menstruation with pregnancy, why does Allah, All-Mighty, bid divorced women to wait as regards marriage for three menstrual periods? Is not one menstrual period enough for them to make sure that they are not pregnant?

The answer to the above question is “No”, simply because cessation of menstruation for one menstrual period is not considered as sufficient evidence of pregnancy. In other words, there are certain cases in which a pregnant woman may, for many reasons, menstruate once, twice or even three times at the beginning of pregnancy. Therefore, the Quranic Verse comes to determine the time necessary to give definitive evidence that a divorced woman is not pregnant. To this effect, Allah, All-Mighty, says,

“And divorced women must wait (as regards marriage) for three menstrual periods; and it is not lawful for them to conceal what Allah has created in their wombs, if they believe in Allah and the Last Day.” [II; 228]

In order to help us know that Islamic Religion is from the Lord, Allah, Creator of all worlds, in order to help us know that such uncommon cases among pregnant women really exist, and in order to help doctors, particularly obstetricians, know such rare cases, Allah, Most Gracious, desires to inform us, through His Divine Book, the Holy Quran, that a divorced woman has to wait for three menstrual periods until it becomes clear whether she is pregnant or not. Therefore, a divorced woman is not proved non-pregnant except after three menstrual periods, when pregnancy, if there is any, is shown clinically and eye-witnessed by both divorced husband and wife.
Praise be to Allah, the Lord of Creations