35% 30% 25% 20% 15% 10% 5% 0%

Chances of Pregnancy by Intercourse Timing

-5         - 4          -3         -2          -1        day of    +1        + 2        + 3                                                           temp rise days before temp rise                                          days after temp rise              Day Intercourse Occurs

Chance of Pregnancy

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Possible Fertile days Non-fertile days

13%

20%

17%

30%*

14%

7%**

0%

0%

0%

Chart information based on research findings by Prof. John Marshall and Dr. J. Barrett

* As you  can see, the chances of pregnancy are greatest two days prior to the temperature rise.   There are several factors that might possibly cause this effect.   First, some women report that their cervical fluid dries up the day that they ovulate.  Lack of fertile cervical fluid can effect conception rates.  Second, some women will have a delayed temperature rise as they are not very sensitive to the thermal effects of the progesterone that is released after ovulation, meaning that it takes 1 or 2 days for the temperature to reflect that ovulation has occurred.  ** There is a smaller chance of pregnancy resulting from intercourse the same day as the temperature rise due to some women ovulating "on the rise,"  meaning that their body basal temperature is sensitive to the thermal effects of the progesterone and rises quickly as the woman is ovulating, instead of waiting till after ovulation has occurred.    Additionally, some women may release a second egg within 24 hours after the first egg is released.   Since the egg can live up to 24 hours, fertilization may take place at that point, although the chance of pregnancy is less than if sperm is present prior to ovulation.   The best chance of pregnancy results from the sperm already being present to meet the egg as the egg is released.

Temperature rise is determined by daily body basal temperatures (BBTs)