Let me try to explain what happens when you try to check a mare past 90 days pregnant, especially if you do it by US.
I have palpated literally thousands of mares in the last 45 years. I have never had an US. I quit breeding for the public about the time I needed to get a US unit. Toward the end of my standing stallions, USing mares was getting very popular. But, I cannot tell you how many mares people had a Vet check (usually by US) before sending them to me to breed that were already bred -- usually that winter before to some yearling or the neighbor's stud or their own colts that had not been weaned yet.
After about 70 or 80 days, a mare's uterus has a big enough embryo in it that it falls over in front of the pelvic rim. This varies in mares with maidens taking longer and older mares with many pregnancies (especially if they are low backed and big bellied) happening earlier. Before that time, US is very accurate as is palpation because the pregnancy is right there. You can run the probe over it and can get your hand around it getting very close to exact date.
As soon as the uterus falls over in front of the pelvic rim, it is inaccessible to a hand or an is difficult to find with an US probe until the foal is big enough to run into and identify head and legs.
So, they can be pronounced 'open' by an inexperienced technician or Vet. I never missed them because I was very experienced and very good at palpations. After the uterus 'disappears', you have to go by the 'lack of a uterus' that you can cup your hand around and you can feel the 'tight' stretched ligaments that go from the ovaries to the uterus. You find an ovary (which should be 'active' size and usually has multiple small follicles on it in a Pg mare) and you follow the taught ligament down from it to where the uterus should be. Then you go across the center and go up the other ligament to the other ovary. The ovaries are lower than in an open mare because they are being pulled down by the pregnant uterus.
I actually have taught some Vets how to palpate mares in that time range because they had only been taught to US. Even today, unless I was doing Embryo Transfers or something like that and I HAD to chose between knowing how to US and knowing how to palpate good, I would chose being able to palpate mares. A good Technician can tell you a lot about a mare. I would rather palpate ovaries for a follicle than US the ovaries and can 'feel' if a mare should be treated or cultured or medicated before breeding better than most people that US them. I can 'feel' uterine tone and condition far better than an US can.