First time parent. The size is still between 5th and 95th percentile, but I am wondering what this CRL figure entails? What can this affect for the baby when he/she's born?
2 Answers
In statistics, the "median" means the 50th percentile - i.e., half of the population are smaller and half are larger. Being smaller than the median is always a common thing - half of us are! And even being, say, at the 25th percentile, which sounds "bad", is not a problem - the 25th percentile is often quite close to the 50th in measurements like this (though I couldn't find a good resource for CRL to verify).
As your fetus, and then baby, grows, it's common to check constantly against measurement benchmarks and be concerned if your child is not in the top range. The best strategy here for managing that anxiety, from what I've found, is simply to keep track of the measurements - you'll see over time that they vary quite significantly.
Always talk with your doctor about these things if they're concerning, but generally don't worry about things like "below median" or "above median" - your pediatrician will know when the measurements are concerning from a health perspective.
If you're asking more from a "does this say how tall my child will be", the answer is "maybe"; CRL was shown in a study to be correlated to midparental height, which itself is correlated to how tall your child will likely be. However, this is fairly uninteresting - what it means is that you can already tell about how tall your child will be, just by averaging mom and dad's height, and either adding or subtracting 6.5cm (+6.5cm for men, -6.5cm for women; 13cm is the average height difference between men and women), and that the CML, and later more accurate FL (Femur Length), already show that.
Do remember though that gestational age at 13 weeks estimated is fairly inaccurate - it could easily be off by several days in either direction, or more, and at 13 weeks, 3 or 4 days of growth is a huge amount. By 37 weeks, it's less meaningful, and the doctors usually have a better idea of gestational age from what I've seen by that point anyway (from other measurements). So, I wouldn't use the CRL at this point to guess your child's future height! Instead, look at mom and dad's height, or just... wait and see :)
Just because lots of things can be measured precisely, that doesn't mean that every value of every measurement is meaningful. IANAD, but there's no reason to think that being slightly below the median has any clinical relevance at all.
And it looks like you are only slightly below the median
https://en.wikipedia.org/wiki/Crown-rump_length
What can this affect for the baby when he/she's born?
You know how many people know the day they had their IVF embryo implanted? They still can't predict to the day when they give birth! Not to mention some people give birth way too early, and way too late. These things can't be predicted.
