Ah yes, the highly anticipated ultrasound appointment that leads to confusion and worry.
Unnecessarily I might add.
Did you get an early ultrasound? By early I mean in your first trimester (12 weeks) of pregnancy. If so, then your due date or EDC (estimated date of confinement) is nailed down. That is the non-invasive gold standard of dating a pregnancy. Our LMP dates can play into the dating a bit. But the problem with those is that not every woman has a predictable cycle. And not every woman keeps a perfect log of dates, add to that spotting that can happen with implantation…you get the picture.
Why is first trimester dating so accurate? Well, the first twenty weeks of pregnancy follows a very predictable growth cycle. All babies grow at the same rate. The technician is able to measure the thigh bone, or femur, and diameter of the head and:voila! An accurate date.
After twenty weeks genetics kicks in. Now you should start eyeballing dad and discussing his birth weight with his mom. Up until now the size of the fetus of the tiny Vietnamese lady and her small stature husband is identical in size to the 6+foot tall Norwegian couple’s. After that twenty week point mother nature takes over. There is no more ‘predictable’ rate, so the dates are going tobe off. The dates can be 2 weeks off for the second trimester, and up to three for the third.
So don’t let your “new” due dates confuse you. The only time they’re significant is if you’re being followed for a growth lag which could be caused by a not so great placenta, high blood pressure and other high risk conditions. You’ll know if your doctor is concerned about any of these things-you’ll be the first person she tells!
I hope this shines a little light on a confusing subject. Please let me know if I can clarify any of my clarifying!