The luteal phase is fixed at 14 ± 2 days and the follicular phase is not, so ovulation is 14 days before the next period rather than 14 days after the last one. Set a cycle length, walk the days, and watch FSH, LH, oestradiol and progesterone move the endometrium and the basal temperature. Then open the tract and date a pregnancy with the arithmetic on show.
Drive the cycle
28 days
Normal is 24 to 38 days. Outside that range it is abnormal uterine bleeding rather than “just irregular”.
Day 1
Day 1 is the first day of full flow, not spotting.
Curves are shaped to show the sequence and the timing relationships, not laboratory concentrations. Read the shape and the order, not the height.
Click a structure. Each one is here because something goes wrong with it in a later chapter.
Leave the ultrasound field empty if there has not been one. Crown-rump length in the first trimester is accurate to about ±5 to 7 days.
Day 1Menstruation — and the follicular phase has already begun