Babies are built like houses. You have to start at the foundation and go in a certain order. You can't put up walls if you don't have your foundation laid. You can't put on a roof if you have no walls. See what I mean?
Growing a baby and a placenta are like this...the same steps have to occur in the same order in all pregnancies. Because of this, ultrasound can give us an idea of how pregnant you are.
An ultrasound done in the first trimester has a margin of error of about plus or minus (+/-) 5 days -- which means somewhere in a 10 day span is your due date. So if they say you are due September 15, that could be 5 days either way (10th to the 20th). Remember: just like all the tomatoes on your plant aren't ready at the same time, babies come early and babies come late. But we know the date given in the 1st trimester is the most accurate because the business of building a baby is done in the same order, and the construction looks about the same across the board, so we expect to see similar timelines for everyone at this point.
In the early part of the second trimester, the margin or error is about +/- 8 days. This means that September 15th date is now widened to September 7th to September 23rd...again, accounting for variation in baby-readiness.
But, just like rain or lack of supplies can make building a house take longer, or a sudden addition of extra workers can make building a house go faster -- the third trimester is where we see this happen. This is why the third trimester is the least accurate when it comes to dating the fetus by ultrasound.
The margin of error in the 3rd trimester is about +/-22 days! This means that September 15th date now expands to August 24th to October 7th! This is why, if a care provider changes a due date in the 3rd trimester, we should really ask what evidence is being used to make that decision.
The third trimester is where we see genetics come into play. If you are tall, your baby may measure big because their leg bone is long, like yours. If your partner has a big head, your baby may be estimated older because their head measures bigger than most of their unborn peers. The variations in physical traits are like the roof coming on sooner, or later, and the actual age of the baby getting lost in the ways we measure babies (head circumference, abdominal circumference, arm bone, leg bone, etc.).
So, all of that to say, in the first trimester you have the closest, most accurate dating of how pregnant you are.
Read more about Pregnancy Dating...
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