Why are the number of days noted and important in determining fetal age? how are the number of days determined? (e.g. 11weeks + 4 days).

LMP: Fetal age is determined by your last menstrual period (lmp). So day 1 is actually the first day of your lmp, not the day you conceived as many think. So the 40 week due date is actually forty weeks from your last period.Woman with regular periods, this is fairly accurate. If you don't know the date of LMP or if your cycle is very irregular, these dates may be off, & your due date may be adjusted.

Rarely useful: For most of pregnancy, the number of days are not important (or accurate). The accuracy of dating is rarely better then +/- 4 days. Yet Obs still write 15 6/7 weeks as if we knew the precise moment of conception (we don't). There are times when we use the number of days. Someone might be admitted at 33 6/7 and discharged at 34 2/7. Or we might want to induce labor towards the end of week 38.