I just found out today that one of my fellow surrogates is pregnant with TRIPLETS. Like most of us, she received two 5-day blastocysts to boost the odds of achieving a pregnancy. However, not only did both blasts implant, but one of them split! She's carrying identical twins and a fraternal sister - all girls! So far all three babies are developing well. :-)
I'm aware that this is a possibility for me too; although I'd thought it "couldn't" happen, obviously it sometimes does.
Ultrasound on June 3rd.