A full pregnancy lasts 280 days, counted from the first day of the last menstruation…

The inner cell mass forms three germ layers. The ectoderm forms the skin and the nervous system. The mesoderm forms the muscles, the skeleton, the heart and the blood. The endoderm forms the lining of the gut and organs such as the liver. The heart forms after one month. A full pregnancy lasts $280$ days, or $40$ weeks, from the first day of the last menstruation. The expected date is the date of the last menstruation plus $280$ days.