Probably the most important thing we do on the first visit if establish your due date. This leads to a lot of questions often as there are several ways to determine the established due date (or EDD) and these often don’t agree so we have to determine which is the most accurate.
It’s helpful to know the first date of your last menstrual period (or LMP) if you know or track your periods. Dating by this method involves putting the date into an app or calculator or pregnancy wheel and it spits out the due date. This is a simple calculation based on assumptions that you conceived exactly 14 days after the first day of your period and that your due date will be 40 weeks after that first day.
Every time your have an ultrasound it will also give an estimated due date as one of the results. Ultrasound typically has approximately a 10% margin of error throughout the pregnancy and is therefore much more accurate in the 1st trimester.
1st Trimester ultrasound is best done through a vaginal probe. This allows the tip of the probe to be placed as close as possible to the womb and gives the sharpest and clearest pictures of the uterus, the gestational sac, baby, ovaries and tubes. This method is also important as it is how we verify that the pregnancy is in the right spot, that there are no worrisome cysts, verify the heartbeat, and also how we make sure there isn’t another baby hiding in there.
Typically we will default to using your LMP if you are pretty sure of the date and if it fits within the margin of error of the ultrasound. If you are unsure of LMP and/or there is a large discrepancy between your LMP and an early 1st trimester ultrasound, we will usually use the ultrasound date. 1st trimester ultrasounds are most accurate between 6-12 weeks.
Why the difference? Most women don’t have exactly 28-day cycles or ovulate, fertilize and implant on exactly the same schedule.
What if you know exactly the date of intercourse and it doesn’t match up? You can imagine the concerning looks that this generates, especially with so many spouses working off on 1-2week intervals and being pretty certain of the date of last intercourse. Sperm can live inside for up to one week after intercourse and can fertilize any time during that interval. This fact often explains the discrepancy.
For IVF and fertility patients undergoing IUI, we will typically use the date of fertilization as the date of conception and add 38 weeks to estimate the EDD.
Once the EDD is estimated, it doesn’t change. Later ultrasounds may show a different due date but these have more to do with the size and growth of the baby.
Your actual delivery date will most like vary from the EDD. The EDD is just an estimated point in time by which we can judge the progress of the pregnancy. Scheduled repeat c-sections mostly occur at 39 weeks or 1 week prior to the due date. Most first-time mothers will spontaneously labor slightly after the due date.
Kyle P. McMorries, MD, FACOG