Pregnancy is caused as a result of sexual intercourse only if it occurs within one's so called "fertile period". The fertile period of a woman is determined by analysing her menstrual cycle with an ovulation calculator or fertility chart. Normally, the menstrual cycle is a 28 day cycle. During the 14th day after a period, the ovum is released. This process is called ovulation and the days 12 to 16 days after a period are said to be a woman’s most fertile period.
An emergency pill or ECP is a birth control pill that can prevent pregnancy after an unprotected intercourse. People sometimes call it the "morning after pill." But you don't have to wait until the morning after sex to take it. In fact, ECP is more effective the sooner you take it. It is a one-dose regimen: you take one pill. The pill contains 1.5 milligrams of levonorgestrel, which is used in lower doses in many birth control pills. The recommended window of protection is 72 hours from the last unprotected intercourse, and the number of acts do not matter. The EC pill does not protect you from pregnancies resulting out of any future unprotected intercourse(s); thus the protection is of retrospective in nature and not prospective.
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