Questions & Answers

Your babies are definitely not 13w size. But there is still plenty of hope! The most obvious explanation is that your dates are off. With multiples, your HCG rises much faster and could cause a pregnancy test to test positive before you are even 4 weeks along. (Happened to me with my twins). And some ladies ovulate later than they think. However, that only accounts for at most a 2 to 3 week difference in size. A 5-week difference should be discussed with your doctor. Fortunately with triplets, you'll be frequently monitored, so you don't have to wait long to get an OK that things are going well.

RE kitten90: Although multiple tend to be smaller WHEN BORN, the increase in hormones associated with multiples actually causes them to grow much more rapidly than singletons in the first trimester. (Their growth slows down in the 2nd and 3rd trimester compared to singletons and multiples tend to be born earlier).

They say normally the CRL in centimeters plus 6.5 centimeters equals gestational age... your triplets according to that method are measuring about 8 weeks.
Although, I think this method is for single pregnancies and not multiples. Twins or triplets tend to be on the smaller side usually.
Did you doctor seem concerned or suggest a follow up exam? If not I think they are probably doing just fine. =) Bless you and the triplets! They are indeed a blessing <3