Explain why the fertile window starts several days before ovulation occurs.
Fig. 4.1 shows how basal body temperature and the concentration of luteinising hormone, LH, changed over one menstrual cycle in a woman. Basal body temperature is the body temperature measured just after waking in the morning.
On Fig. 4.1, draw a curve to show how the concentration of progesterone in the blood changes during this menstrual cycle.
The follicular phase of the menstrual cycle starts when menstruation begins, and ends when ovulation occurs. Using Fig. 4.1, suggest when the follicular phase started and finished in this menstrual cycle. begin ................. ended ..................
Three methods a woman can use to work out her fertile window are: method 1: using the date on which each menstruation starts to predict when ovulation will happen method 2: using disposable urine dip sticks to measure the amount of LH breakdown products in urine (the greater the LH in the blood, the more breakdown products are found in urine) method 3: wearing an electronic device in the armpit that measures body temperature continuously.
Suggest why using method 1 on its own is unlikely to be a very dependable way of avoiding conception.
Explain how method 2 could be used to avoid conception.
Suggest why method 3 is likely to be a better predictor of ovulation than taking basal temperature with a thermometer each day.
An investigation was done into the timing of the fertile window. It involved women who were trying to become pregnant. Urine samples from each woman were tested every day for several months for LH breakdown products. The women noted the days when they had sexual intercourse, and also the days when menstruation began. $136$ of the women became pregnant during the study. The results were used to calculate the probability of a woman being in the fertile window on each day of her cycle. The results for women with regular $28$-day cycles are shown in Fig. 4.2. Discuss what these results suggest about the guidance that the fertile window lasts from day $10$ to day $17$ of the menstrual cycle.