During twin pregnancy from 12 to 16 weeks your twins are still growing at a fast pace. They have also graduated to the fetus stage, although, many of us twin moms are already fondly referring to them as "Baby A" and "Baby B". The kidneys are beginning to function and by the 16th week water in their stomachs and urine in their bladders may be detected via ultrasound. They are actually contributing to the amniotic fluid by swallowing it and then expelling it back out through urination...Their first contribution to their environment! They will measure close to 6 whole inches by the end of the month. There is more movement going on too, though unlikely strong enough to be felt by the mother.
Some women who are expecting twins will have to start wearing looser clothing at this point. The uterus is growing right along with your two babies, so it may already be exhibiting it's superior size when compared with singleton pregnancies, though everyone will measure a little differently at this early stage between 12 to 16 weeks.
An escalation in headaches is a common grumble, as well as the start of constipation. Luckily, this is when morning sickness usually starts to diminish, so hang in there!