Week 1 week2 week3 week4 week5 <span><span>The "weeks" refer to embryonic development and differ from clinical weeks (shown in brackets, from the first day of the Last Menstrual Period, LMP or GA)</span>The "stages" refer to Carnegie stages of development.<span>The "timing" refers to days from fertilization or post conception age (PC), not the clinical or gestational age (GA) calculated from LMP (add 2 weeks).</span>The dates and staging are also "ideal", and there is significant biological variability in the general timing of events.<span>Week 1 to Week 8 are considered the embryonic period of development.</span><span>Week 9 to week 37 or birth are considered the fetal period of development.</span><span>The first month after birth is the neonatal period of development.</span></span>