in islam hw is the age of the pregnancy calculated to determine how many days old the fetus is? is it from the first day of the last cycle, the dating scan or when the couple had intercourse?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

There is no specification in the Sharee’ah about the calculation of the age of the fetus. This matter is determined by the doctors specializing in this field. Indeed, the Advisory Section of Islamweb received a similar question and one specialist doctor said that the period of pregnancy is calculated from the first day of the last monthly menses. This can be determined by using modern equipment like the ultrasound equipment and the like. For more benefit, please refer to Fatwa 116788.

Allaah Knows best.