Hello, I just joined and was reading this thread since I am in a similar situation.
My wife and I got a mare a few weeks ago and was told she pregnant and should drop maybe April to June but she didn't look it, the vet didn't really look her over and said he would bet she isn't.
However, since then we have checked her out much closer and believe she is.
Her teats have enlarge a small amount ans her belly is growing very wide but not dropping down yet.
I was hoping for an update on yours and wish you the best.
P.S. This our first time to deal with this as well even though I grew up with horses we didn't breed them.