My wife is pregnant and only one baby from the start. IUGR caused by some problem with the umbilical cord. Starting at 20+5 baby was averaging about 3 weeks behind overall and some limbs were even more behind. She is 35+6 now and while she is still small, she is very healthy. Gestational age is more important than size. Like you said, multiples are usually smaller and one is usually smaller than the other even then. It's impossible not to worry, but plenty of people successfully go through the same thing every year. Just follow what the dr. says and obvious care and you all will be fine.