Gestational Age

Last menstrual period Due date calculator and Ultrasound expected delivery date calculator. Gestational age calculator. To be used by Gynecologists and Obstetricians, but so easy that pregnant womans can use it too.
When you start announcing your pregnancy, everyone will start asking what your due date is. While this date can help you plan your life around your birth, there is no precise way to tell for sure what day your baby will be born. Every pregnancy's due date carries a plus or minus two weeks. In fact, less than four percent of women actually deliver on their due dates.
Naegele's Rule is a standard way of calculating the due date for a pregnancy. The rule estimates the expected date of delivery (EDD) by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP). The result is approximately 280 days (40 weeks) from the LMP.
LMP may not be the best date to use as the basis of a due date calculation, but it remains popular because few women know exactly what day they ovulate or conceive a pregnancy, and because no algorithm can predict the exact day that spontaneous labor will occur no matter what considerations are taken into account.
Ultrasound confirmation of gestational age

Since the 1970s ultrasound scans have allowed measurement of the size of developing embryos directly and so allow for an estimation of gestation age. Ultrasound dating is most accurate if undertaken in the first trimester (first 12 weeks of pregnancy) with a 95% error margin of 6 days. Scans performed in the second trimester have an error margin of 8 days and those in the third trimester a margin of 2 weeks.