When Are you Due? Pregnancy Dating Explained

“When was the start of your last menstrual period?” If you are a uterus possessing person in the 21st century, you have probably been asked this question A LOT at doctor’s visits, and it may have often seemed unnecessary. For the purpose of pregnancy dating- this question is actually important. Your estimated due date (EDD) is calculated by adding 40 weeks (280 days) to the first day of your last menstrual period (LMP). This is the day your last period started.  This method of pregnancy dating is based on a “typical” 28 day menstrual cycle.  It estimates that ovulation and subsequent conception takes place about 14 days after the start of your last period. So you are technically not pregnant for the first two weeks of a 40 week pregnancy! There are limitations to accurate pregnancy dating based on LMP, however, since many ovulating people do not have a 28 day cycle, or a regular cycle. Because of this, early ultrasound is used to confirm pregnancy dating. A “viability” ultrasound is an ultrasound performed early in pregnancy, usually between 6-12 weeks, that confirms a pregnancy is growing in your uterus and uses measurements of the developing fetus to estimate how far along in pregnancy you are. It is also used to determine if you are expecting a single baby or multiples (surprise-it’s twins!). Folks who are tracking their cycles and ovulation may have additional information- such as the date of conception- that healthcare providers can use to accurately estimate their due date. 

Pregnancies conceived using reproductive technology often have very exact dating because we know when an embryo transfer or insemination occurred. In IUI pregnancies, the gestational age is typically calculated from the date of the insemination procedure. In IVF pregnancies, the gestational age is calculated from the date of embryo transfer. Even with IUI and IVF pregnancies, early ultrasound will likely be used to confirm and refine the estimated due date based on fetal development.  Your gestational age will be calculated in weeks and days and will increase each day as your pregnancy progresses. For example, your pregnancy will have come to term when you are 37 weeks and 0 days pregnant. Accurate pregnancy dating is important because your gestational age will be used to guide the timing of pregnancy care and will impact the interpretation of certain prenatal tests. 

TAKE AWAY:

You can calculate your due date by adding 280 days to the first day of your last period. Early ultrasound and timing of infertility treatments provide precise pregnancy dating.