The effectiveness of this method depends entirely on careful
recording of the menstrual cycle. Intercourse is avoided during the
middle 10 days of the cycle, when conception is most likely, but
this varies, from woman to woman. If you have an irregular cycle, it
may be difficult to calculate the exact 'safe-time'.
Rhythm method calculations:
Calendar method
Counting the first day of her menstrual period as day 1, the woman
keeps a record of her menstrual pattern for the last three to six
months, or longer if possible. She then calculates her 'safe period'
by recording the number of days in her shortest cycle, and subtracts
18. She then records the number of days in her longest cycle and
subtracts 11. Thus if her longest cycle runs on a 30 day schedule
she will after subtracting be left with the number 19, and the
number 10 if her shortest cycle is 28 days. This means that her most
fertile period, which would obviously be 'unsafe' would be the days
between 10 and 19. This is not a recommended method as it may fail
if something such as stress alters the length of your cycle.
Basal body temperature method
The woman takes her temperature every morning upon waking, with an
ordinary or specially designed thermometer and records the taking on
a chart. From day 1 of your period, until around ovulation, your
temperature will be 0,3 - 0,6°C lower than the time between the end
of ovulation and your next menstrual period. Intercourse should be
avoided three to four days before and after this temperature change,
as that is when you are likely to be at your most fertile. This
method can be unreliable especially if your cycle is upset for
whatever reason or an illness affects your temperature reading.
*1 to 20 women out of every 100 become pregnant while using Natural
Family Planning methods.*
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