The fertile phase is between the 11th and 17th day, in that day, the fertilisation can occur. This period is called the fertile phase.
An ovum in a female body can live for 24 to 36 hours only. Because the ovum will be released in the middle of the menstrual cycle; about the 14th day, this means that fertilisation can occur between the 11th and 17th day to produce a zygote if the sperm enters the Fallopian tube.