A pregnant woman is most excited to know about her pregnancy due date when she will bring her bundle of joy in the first antenatal visit.
Why do I need to calculate my due date?
It is important to calculate the gestational age and the due date accurately so that the necessary tests can be done at the correct time, the appropriate obstetric care can be given, and interventions such as vaccinations or inductions can be planned appropriately.
How to calculate your due date?
The two methods for calculating the due date are the from date of last menstrual period (LMP) and the ultrasonography.
Pregnancy Due Date From LMP
The first step is to determine the LMP. In a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day then the EDD is 280 days after the first day of the LMP. But this does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation.
Ultrasonography
In First trimester
This is the most accurate and can give an accurate gestational age and EDD even if the LMP is not known.
Up to 13.6 weeks, gestational age assessment based on measurement of the crown-rump length (CRL) has an accuracy of ±5–7 days. The measurement of the CRL is more accurate earlier in the first trimester that ultrasonography is performed.
If ultrasound-based EDD before 14 weeks of gestation differs by more than 7 days from LMP based EDD, then the EDD from the ultrasound is accepted as the correct date.
In the Second Trimester
If the first ultrasound is done after 14 weeks, then it has much less accuracy. The date from the first-trimester ultrasound is considered final in case the date from the second-trimester ultrasound is different.
Gestational age assessment by ultrasonography from between 14 weeks to 22 weeks of gestation, has an accuracy of 7–10 days, and from 22 weeks to 28 weeks, it has an accuracy of ± 10–14 days.
In the Third Trimester
Gestational age assessment by ultrasonography in the third trimester that is after 28 weeks is the least reliable method, with an accuracy of ± 21–30 days.
It will need to be repeated in two weeks to see interval growth and then decide about the correct gestational age in case there is a discrepancy.
Special situation – in case of assisted reproductive techniques ART.
If a pregnancy resulted from ART, then the gestational age and EDD is calculated from the age of the embryo and the date of transfer.
