I had a doctors appointment today and my doctor informed me that my sons umbilical cord only has two…

I had a doctors appointment today and my doctor informed me that my sons umbilical cord only has two vessels instead of the 3 like normal. has anyone else experienced this. we have already had our down syndrome test. and it was negative. the doctor didn't seem to concerned he just let me know he could be small and I would have to get more frequent ultrasounds.