NT scan with weird outcome (long... kinda pointless)

BellaOso

I have no idea what I want from this post, so please feel free to tell me your story or experience. (I will say, I really like my OB, his office, his partners and his staff. I went to an imaging facility that they use as a referral.)

I had my NT screening today. I am supposed to be 13 weeks and like 5 days. (Due August 7 if anyone wants to do the actual math.) This was based on the initial ultrasound at 7 weeks. We could not use the date of my LMP because I was postpartum. My last and only MP since giving birth to DD in March was Sept 1st... That is not helpful... 

Today, the tech did her screening and was pretty terse, saying the doctor would address the findings with me. My husband and I just kind of watched, and over and over again when she took a measurement, the returning result showed me being between 14 weeks 5 days and 15 weeks. (Due date of July 29.)

When the doctor who reviews the images came in she said, "We can't tell you anything. As far as we call tell, you are too far along. Everything looks good as far as limbs and stuff but we can't complete the NT portion of your screen. I'll forward these along to your doctor."

So... I guess I am like 9 days earlier than my OB thought? Or the baby is just measuring ahead? I'm going to contact my doctor tomorrow. I realize being a little further along is not even remotely a big deal... except I didn't get this test. I wasn't to concerned about it anyway, but it does affect some things, like which due date will they chose to use for overdue stuff, like inducing if you go over 14 days? And isn't the measurement my OB took at 7 weeks more accurate than the one now? 

I realize that this may sound like I am stressed out by I assure you, I'm not, like at all, just kind of confused? Wondering if this happens frequently?