Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 25The Gonads: Development & Function of the Reproductive System 415

women. A few pregnancies resulted from isolated intercourse
on day 3, 4, or 5 before ovulation, although the percentage
was much lower, for example, 8% on day 5 before ovulation.
Thus, some sperms can survive in the female genital tract and
fertilize the ovum for up to 120 h before ovulation, but the
most fertile period is clearly the 48 h before ovulation. How-
ever, for those interested in the “rhythm method” of contra-
ception, it should be noted that there are rare but documented
cases in the literature of pregnancy resulting from isolated
coitus on every day of the cycle.

The Estrous Cycle

Mammals other than primates do not menstruate, and their

sexual cycle is called an estrous cycle. It is named for the con-

spicuous period of “heat” (estrus) at the time of ovulation,

normally the only time during which the sexual interest of the

female is aroused. In spontaneously ovulating species with es-

trous cycles, such as the rat, no episodic vaginal bleeding oc-

curs but the underlying endocrine events are essentially the

same as those in the menstrual cycle. In other species, ovula-

tion is induced by copulation (reflex ovulation).

FIGURE 25–25 Basal body temperature and plasma hormone concentrations (mean ± standard error) during the normal human
menstrual cycle. Values are aligned with respect to the day of the midcycle LH peak. FSH, follicle-stimulating hormone; LH, luteinizing hormone;
M, menses.

̊C 36.6

36.8

36.4

20

80

0

40

60

50

0

100

150

200

Follicular phase Luteal phase

Basal body

temperature

Inhibin A (pg/mL) Inhibin B (pg/mL)

20

50

0

10

30

40

500

0

1000

1500

Progesterone (nmol/L)

Estradiol (pmol/L)

20

40

0

10

30

5

0

10

15

LH (U/L) FSH (U/L)

-14 -7 0 7 14

Days relative to midcycle LH peak

M