Most people talk about pregnancy lasting for nine months, but we
generally measure pregnancy by weeks, lasting 40 - 42 weeks from conception. Your doctor or midwife will calculate the date you can expect your baby to be born based on the first day of your last menstrual period. They base it on you having a regular cycle of 28 days.
To calculate your expected date of delivery (EDD) add seven days and nine months to the first day of your last menstrual period. For example, if the first day of your last period was on the 2nd of March, your expected date of delivery (EDD) is the 9th of December. Most women will go into spontaneous labour between 37 - 42 weeks of pregnancy. There is no way to accurately predict the exact delivery date; we know that first time mothers are more likely to go past 40 weeks.