The following explanation is from Shaikh GF Haddad's Hadiths on the Formation of Human Life.
THE ORGANOGENESIS HADITH
Imam Muslim narrated from Hudhayfa ibn Asad that the Prophet Muhammad said - upon him and his House blessings and peace:
After the sperm-and-ovum drop (nut.fa) has been [in the uterus] forty-two days, Allah sends it an angel that gives it form and fashions its hearing, sight, skin, flesh, and skeleton.
The time frame given cited above is in conformity with embryological observation. The embryo reaches the sixth week without showing the semblance of human form but by the seventh week of its life - about three centimeters in size and beginning to move - that semblance becomes visible in the formation of the essential organs including the sensory organs and grown bone tissue. The arms and legs have lengthened. The foot and hand areas are distinguishable and they have digits. The first recordable brain wave activity occurs.
This organogenesis peaks precisely at 42 days.
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