How is the age of the foetus worked out?
Question

We know that abortion of the foetus before forty days is permissible, if there is a medical reason or there is a risk to the mother from being pregnant. But how are the forty days counted? Is it from the time of ovulation or from the time of the last menstrual period?
Praise be to Allah.Firstly:
We emphasise that we should not be lenient at all with regard to abortion in the first forty days, unless the conditions are met which have been affirmed by fiqh councils, senior scholars and fatwa committees. That has previously been discussed in detail on our website. Please see questions no. 115954 and 171943 .
Secondly:
We asked Dr. Muhammad ‘Ali al-Barr (may Allah preserve him), a consultant of internal medicine and a consultant in the Islamic Medicine Department at the King Fahd Centre for Medical Research at King ‘Abdul ‘Aziz University, and a medical expert in fiqh councils about this matter, and he replied:
According to doctors, the calculation of pregnancy begins from the first day of the last menstrual period before pregnancy occurred. This is used as a standard to count the weeks of pregnancy, which are forty weeks. But the age of the foetus begins from approximately two weeks, between 13-15 days, after the end of this menstrual period. This is the time when ovulation takes place, which occurs after the end of menses, and that only happens once after the monthly period has ended, and after that the [fertilized] egg settles in the uterus within only 12-24 hours, which is when pregnancy begins. So the real age of the foetus begins 14 days after the end of the previous menstrual cycle. This is what is the case for most women. End quote. (Recorded answer).
Based on that, working out the age of the foetus begins fourteen days after the end of the menstrual cycle that preceded the pregnancy, which is the time of ovulation.
This is the basis on which the counting of forty days, and when the soul is breathed into the foetus are worked out, and it is used to determine the ruling on bleeding that occurs after miscarriage, if it occurs before or after eighty days.
And Allah knows best.