Some try to get, others to get not pregnant
Periodic abstinence is based on the menstrual cycle. Sexual intercourse is confined to the barren period. For at least 3 months, the (regulary?) cycle of 28 (24 to 32) days is observed. He starts with the menstrual bleeding. Ovulation occurs 14 days before the bleeding (day 14, or between the 12th and 18th day). This depends on the length and regularity of the period, and is for every woman different.

An egg may be fertilized 24 hours. Sperm can still perform fertilization 2 to maximum 6 days after ejaculation. This creates a fertile period of 7 days before to 2 days after ovulation.

It goes without saying that this method is not foolproof, not even with a safe margin.

More accuracy you get by a combination with the temperature method.
The body temperature goes after ovulation about 0.2 to 0.9°C up until the next menstrual period. Notice the temperature in the morning a number of months before getting up at a fixed time to obtain a more accurate idea about the time of ovulation which causes the temperature rise.From the second day with an elevated temperature is temporarily no longer the occurrence of pregnancy possible.

Coitus interruptus is interrupted intercourse. The penis is withdrawn from the vagina before ejaculation occurs. Will there so called 'precum' come in the vagina, it can still lead to pregnancy, since this can also contain already active sperm. This method is also called withdrawal method.

Other tools
Condoms used to be made from sheep intestine. (The sheep was dismissed...)

Babies are rarely more than 100 years. (J. M. Gourio)
Children are literally relationship gifts.