The most accurate dating we can use involves the day for either IVF or embryo transfer as this is a certain date. In fact, we can even know the specific time of the procedure. After IVF, the next most accurate criteria to use is a first trimester ultrasound (prior to 14 weeks) that shows a fetus with a heartbeat (one with just a sac and no fetus isn’t as accurate). It is accurate to within 5-7 days. As we move into the second trimester, ultrasound can vary as much as 2 weeks from the actual due date, and in the third trimester, we can variation from 21-30 days. This can be a full month discrepant from when a patient is due!
If you think about it, this totally makes sense. If asked to compare a bunch of 6 month olds to determine their age, you’d probably be within a few months of their actual age. Looking at 6 year-olds, you might be within a year or two. With 16 year-olds? Good luck! You’d probably be fortunate to be within 5 years for some of them. For babies in-utero, the same holds true. The earliest estimates of age are going to be the most accurate, so that’s what we go with. The due date at each ultrasound is the average due date for a baby of that size.
So, even if your baby grows faster than average, that doesn’t mean we get to deliver early. There are still developmental milestones for the brain and lungs and other parts of the body that we’d you’re your baby to reach before delivery. To put it in perspective, you wouldn’t expect a 12 year-old to graduate high school just because he’s now 6 feet tall. Whether the biggest kid in 6th grade or the smallest in 6th grade, they are still 6th graders. They’re probably not quite ready for the college application process just yet 😊
Interestingly, I’ve yet to have a patient ask me to push back their due date. No one’s ever said to me, “You know. I just love being pregnant. Do you think you could move my due date back a week or two? Maybe a month?” In fact, it’s more the opposite. I can almost hear the nervousness of those whose baby is growing a little slower than average, and their thoughts seem to be going a little like this: “Maybe Dr. Denson won’t notice my new due date is later than we thought. If I don’t make eye contact and rush him through the report, then maybe I won’t have to stay pregnant any longer.” Fortunately, for those moms (and the health of the doctors who would have to ask them to stay pregnant longer), pushing back the due date at that point would be like making a smart child stay in kindergarten because he or she is too short to go to first grade.
There you have it. Earlier dating is best! Once you’ve been assigned a due date after an ultrasound showing a fetus with a heartbeat, you can mark it on the calendar, and the countdown can begin! So even though we can’t move up your due date, you don’t have to worry about us moving it back either 😊