Hi thanks for your question.
At 13 weeks gestation, ultrasound
cannot miss twin pregnancy
.There is no such space in uterus
where other baby can hide.Yes it is possible that pregnancy of 16 weeks duration appears to be of 20 weeks on clinical examination by fundal hight.There are few reasons for that.First being the excessive liqour (fluid layer surrounding the fetus).The second reason could be fast growth of fetus,which can be normal or pathological due to variuos reason.I would suggest you to repeat ultrasound to rule out or confirm twin pregnancy (chances of which are less).
Hope this answers your question.