Finding Out Your Due Date
Your estimated delivery date, or due date, is calculated based on the first day of your last normal period (usual amount of days and flow). Based on this information, your due date may be calculated in a few different ways. Your doctor or midwife will start by asking when the first day of your last normal period was. If your last period was different than what you normally experience (shorter, less bleeding), you may be asked about your period before that. A due date based on the first day of your last normal period is usually calculated by Naegele’s Rule, n amed after German physician Granz Karl Naegele, who invented it. The rule is based on the assumption that the woman has a normal period every 28 days. Based on this assumption, an estimated date of delivery would be approximately 40 weeks, or 280 days, after the first day of her last normal period. This formula provides an approximate due date, and many babies born in the middle of a ten-day time frame. Even though there has been no formal research done to test it, it is still used today.
The Naegele Formula: First day of LMP (last monthly period) + 7 days – 3 months= EDD (estimated due Date.
Your doctor may also use something called a “pregnancy wheel,” or gestation calendar, like this one:
For the more high-tech doctor or midwife, the computer offers different kinds of software or applications to determine the date as well. Since many versions available online are free, and there are also apps available for smart phones, you could try using them at home, too.
That being said, make sure to visit a doctor or midwife soon. It is extremely important that the estimated due date is correctly determined. A due date that is incorrectly determined may lead to screenings (testing) being performed at the wrong times, incorrect reading of the results of screenings, and incorrect diagnosis. When a baby is born, the gestational age is an important factor in determining the health of the baby The farther along into a pregnancy a woman is at her first prenatal visit, the harder it is to accurately determine a due date.
I don’t remember the last date of my period!
If you are unsure of when the first day of your last period was, don’t worry. An ultrasound can be performed to determine gestational age (how many weeks into the pregnancy you are). Based on this information, an approximate due date will be determined.
Determining an Estimated Date of Delivery by ultrasound in the first trimester
It is possible to become pregnant even though a woman has not had a period for a long period of time. For instance, if you are breastfeeding, it is common not to have a regular period, but it is still possible to become pregnant. In other cases, women may not have a regular monthly period.3 In situations such as these, determining a due date based on the first day of your last regular period will not be accurate. In order to confirm a due date, an ultrasound will be done.
The ultrasound may provide a due date that is different than what was determined by your last monthly period (LMP). The ultrasound will be the most accurate if is done while you are still considered to be in your first trimester (first 13 weeks).4 During the first trimester, a due date is determined by measuring the distance between your baby’s crown and rump, which is from the top of the head to the buttocks. The due date that is determined by the ultrasound may be used instead of the due date that was determined by your last period if the difference between the two dates is greater than seven days. Your physician will compare the two, then decide what method is the most reliable.
Determining an Estimated Date of Delivery by Ultrasound in the Second Semester
If your first prenatal visit isn’t until you are already into your second trimester (based on the first day of your last period), the ultrasound will be performed to measure your baby’s head circumference (size of the head), length the baby’s femur (thigh bone), size of the abdomen (belly), and the biparietal diameter (the transverse diameter of the head), will be used to determine a gestational age. 4,5
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- Lynch, C., & Zhang, J. (2007). The research implications of the selection of a gestational age estimation method. Pediatric and Perinatal Epidemiology, 2(1),86-96. doi: 10.1111/j.1365-3016.2007.00865.x
- Hunter, L. (2009). Issues in pregnancy dating: revisiting the evidence. Journal of Midwifery Womens Health, 54(3), 184-190. doi: 10.1016/j.jmwh.2008.11.003
- Hadlock, F., Deter, R., Harrist, R., & Park, S. (1984). Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology, 152(2), 497-501. doi: http://dx.doi.org/10.1148/radiology.152.26739822
- Kalish, R., Thaler, H., Chasen, S., Gupta, M., Berman, S., Rosenwaks, Z., & Chervenak, F. (2004). First and second-trimester ultrasound assessment of gestational age. American Journal of Obstetrics and Gynecology, 191(3), 975-978. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15467575
- Committee Opinion 611. Method for estimating due date. (2014). The American Congress of Obstetricians and Gynecologists. Retrieved from http://www.acog.org/Resources-And-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Method-for-Estimating-Due-Date