Deduct three months from the last day of your menstruation, add 3 days to it, and add 266 days which is roughly your date of conception. There are many online easy-to-use mortgage calulator available for the same purpose; a simple search will help you with that. A rough idea of the baby's birthdate will help you to prepare for the big day. However, sometimes this will not be accurate as the due date is vastly dependent on many biological factors and varies from person to person. Ultrasound monitoring of the development of a baby is a scientific way to determine the due date. Many doctors advise not going for too many ultrasound tests as sometimes it can harm the growth of the baby. My pick would be the use of online due date calculators, for ease of use, and privacy. You can personalise them as well and your details are securely stored.