This is one of the most common questions I get in the office and I get why: It’s confusing! By convention, we start counting from your last menstrual period (LMP).
👨🏼⚕️For most women, your due date (also known as EDD: estimated due date or EDC: estimated date of confinement) will either be calculated by your LMP or an ultrasound.
📅Using LMP: This is the best measure for those with regular periods who are sure of their LMP.
The easiest way is to google “pregnancy wheel” and use an online one.
For the nerds like me, you can use the Naegals rule (assumes 28-day cycles). LMP (say 9/1/19) -> Add a year (9/1/20)-> Minus 3 months (6/1/20) -> Plus 7 days = EDD of 6/8/20 (6/7 this year due to an extra day on Feb 29).
🖥Using ultrasound: Most accurate in the first trimester. We use this for patients with unsure LMPs, irregular periods, or when the LMP differs from a first-trimester ultrasound by more than a week (because we assume the LMP is wrong).
For people that don’t find out we are pregnant until the second or third trimesters (yes, it happens), we can use ultrasound, but it is less accurate as the pregnancy goes on.
💉For patients undergoing IVF: We assume the day of the retrieval is the day of ovulation. So for example, if you have a day 5 transfer, the day of your transfer is assumed to be 2 weeks and 5 days after your “LMP).
🆒Fun fact, pregnancies are not 40 weeks, they are closer to 37 weeks from implantation to your due date!
🙈If this is overwhelmingly confusing, don’t worry! As a doctor trained for +15 years for this, I have your back!
Any questions?