1. Use Gestational Age
In early pregnancy, the use of ultrasound can effectively and accurately determine one's estimated pregnancy due date. An obstetrician can use ultrasound to visualize a fetus as early as 5 to 6 weeks into a mother's pregnancy. By measuring the size of baby from crown to rump and by looking for the presence of a heartbeat and a fetal pole, an obstetrician can determine the approximate age of the baby or the baby's gestational age. Since a pregnancy lasts approximately 40 weeks, one can then subtract the current gestational age of the baby from 40 weeks. Then, count ahead this many weeks to determine the estimated pregnancy due date. This method works well when a woman doesn't remember the date of the first day of her last menstrual period or when a woman has irregular menstrual cycles. Be aware that only approximately 5% of babies are born on their due dates. Most babies are born between 38 and 42 weeks of gestation.
2. Count Ahead From Your Last Menstrual Period
If you remember the first day of your last menstrual period (LMP), you can easily calculate your estimated pregnancy due date without using ultrasound. Because pregnancies last 40 weeks (on average), you can simply count ahead 40 weeks from the first day of your last menstrual period to determine your pregnancy due date. For example, if the first day of your last menstrual period was July 10, your estimated pregnancy due date is April 17.
3. Count Backwards Using Naegle's Rule
Naegle's Rule allows one to determine a pregnancy due date by counting backwards from the last menstrual period. Simply count backwards 3 months from the first day of your last menstrual period. Then, add 7 days to that result. This will then determine your pregnancy due date. For example, if the first day of your last menstrual period was June 15, you will count back 3 months to get March 15. Adding 7 days to that date gives an estimated pregnancy due date of March 22.
4. Let the Internet Do the Work for You
Most women don't have the patience to calculate their pregnancy due date themselves using Naegle's Rule or by adding 40 weeks to the first day of their last menstrual period. If this describes you, don't worry. You can simply use the Internet to help you calculate your pregnancy due date. Many pregnancy related websites have pregnancy due date calculators online. You can either calculate a pregnancy due date by inputting the first day of your last menstrual period or by inputting your ovulation date. Simply search the Internet for "pregnancy due date calculator" to find a pregnancy due date calculator.
