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conception is most possible during ovulation period.
ovary ovulate follicle at midcycle. which you can count according your cycle.
i.e. ovulation is on 14th days before your menses date. so if your menses is regular then you can predict the ovulation day.
like, in 28 days cycle ovulation is on 14th days (28-14=14)
likewise in 30 days period it is on 16th days (30-14=16) ans so on
better chance of pregnancy
is 3 days before ovulation and 3 days after ovulation. so during this ovulation week (known as danger period) having unprotected sex is most likely to become pregnant.
have a good health