Q. Heather asks, "Is it possible to be pregnant with twins and it show up as a single pregnancy on the ultrasound?"
A. It is possible early in pregnancy, although it's fairly uncommon. However, it definitely can happen -- it happened to me!
I had an early pregnancy transvaginal ultrasound at seven weeks. Here is an actual photograph from that ultrasound. One healthy embryo was clearly identified and I went on my merry way through pregnancy. Then, several months later I went in for a routine twenty week ultrasound scan. Surprise! "You've got two!" (I'll never forget that moment of utter astonishment.)
What happened in this case was that my monozygotic twins were monochorionic, sharing a placenta and encased in a single chorion, the outer membrane of the sac. Basically they were so close together that one is shadowed behind the other. When viewed in a two dimenstional picture, all that is visible is the "top" twin.
So it is possible, just not very likely. Keep in mind that this picture is nearly fourteen years old; ultrasound technology has improved since then, and the rise in multiple births ensures that doctors and technicians are more adept at identifying multiples. Also, this was an ultrasound scan very early in pregnancy; I was only seven weeks along when this was taken. By twenty weeks it was exceedingly and unmistakably evident that there were twins, so a later ultrasound would be more reliable. There were also some other circumstances that kept the presence of twins from being revealed, including a lack of extreme symptoms such as nausea or fatigue and a rotating set of doctors so that no one person listened to the fetal heartbeat(s) more than once. I never measured large for gestational age, and I never had any of the common screenings that might raise a flag for multiples such as hCG or AFP.
Think You Might Be Having Twins?