~pi: you are also an uncommonly well-informed patient, and you speak the "language" that they do to. I guess what I should have said, is that even just asking what the error range in those size estimates is could begin a different convo with your HCP. I definitely noticed that with my more medicalized HCP in the past, when I started asking more technical questions, all of the sudden they told me a lot more (and more accurate) information. And I am not a natural scientist or medical professional, just could understand enough of the technical talk to ask questions about it.
Obviously multiple measurements are much more accurate. But many GD moms will have one "size check" around 36 weeks (along with their 20 week anatomy scan if they had that). At least in my experience that was the recommendation. Certainly if they were talking c-section after one size check, I would be asking for a confirmation, for more information, for a careful look at the error ranges, for a look at the shoulder size, not length, overall size, etc. And in my case I would be touting my history of two uncomplicated vaginal births of large babies, as well as my own and my husband's birth weights (both of us were large- I was over 9lbs, and I think DH was just under). I am working with HBMWs who are very relaxed about it and we all assume I will have a large baby and that this is to be expected and is healthy and normal for me and my baby.
Originally Posted by ~pi
I think this must vary. My team is very clear on the fact that they are inexact estimates, especially when it's just one measurement.
However, when you have them done regularly throughout pregnancy and the babe keeps tracking on the same percentile, it's statistically unlikely that they are off by the same amount, in the same direction, every single time. (My little one has been 80-90th %tile all the way along. High-risk OB has at least been very reassuring. I quote: "Look. Your A1c is better than most humans. Maybe you just grow big babies. You're tall. Try not to worry about it.")
livacreature, in your case, as long as the baby isn't measuring incredibly tiny (which she isn't) and isn't measuring large either (also not) at this point, it's unlikely she's going to move much on the curve, so they may see little point to further size checks. The concern with type 1 is large babies.
As I understand it, the issue with diabetes is that fetal abdominal fat is associated with fat on the shoulders as well. (One of my midwives refers to it as when the baby looks like a little linebacker.) This is thought to be a major factor in the significantly increased risk of shoulder dystocia among diabetics.
I don't know if this applies to GD in the same way as it does to type 1, but shoulder dystocia is one of the major, major issues in a type 1 birth. (I believe it is the same with GD, but I'm not 100% sure.)
I have heard conflicting things about the shoulder size/subcutaneous fat but I am certainly not going to pretend to be an expert on this! And in my case, blood sugar has always been under very good control, so nobody has been concerned about hyperglycemia. Actually the high risk OB who did a consult with me last time looked at my birth history (first child, 8lbs6oz did have a mild SD but it was resolved by a change in my position), and he looked at my blood sugar logs and my "size check" and agreed with all the midwives that I would have a baby that was at the large end of the normal range, and that it would not likely be a problem. I do think he treated me very differently b/c I was just a tiny bit of a PITA. Not confrontational, but I engaged him with my own questions and information.
I still think there is a big difference between a 9lb baby (high end of normal range) born to a woman who is herself genetically large and has had good blood sugar control, and, say, an 11lb baby born to someone who has had uncontrolled GD and is of small stature.
Also various MWs and docs told me that there is also a higher risk of fetal death after 40w and they are not sure why, with GD. But I think many people do not like to discuss that at all, especially not with pregnant moms. So they talk about the shoulder dystocia a lot instead.
Also, mutiple times people asked me if I had GD with my first due to his size. I did not, and was screened three times for it, negative each time. I was really defensive about it at the time, "some people are just big!" My son is still a very large-for-age child (not fat AT ALL, just tall, lean, lots of musculature, also very coordinated. He looks to be a good year or two older than he is). I am not discounting that size can be a problem, but I think there is this hyper-focus on it as a risk factor, tied in part to a culture around a fear of fat.