If you get a transvaginal ultrasound at 6 or 7 weeks, you may see only one baby when there are two. Your twins have the same amniotic sac. Twins may form their own unique sacs at times, but when they share one, the risk of one twin concealing during an early ultrasound increases. The only way to be sure that you're looking at two babies is if you can do a biophysical profile on them both - which means performing special tests on the fetal heart rate during pregnancy. These tests are not usually done until after 12 weeks' gestation because the babies aren't strong enough to survive outside the womb and no treatment options are available for multiple births. At 8 weeks, most embryos have a head, a torso, and two legs. By 9 weeks, most embryos have a clear pair of eyes, a nose, lips, a mouth, and a chest cavity. The skin also develops pores that will eventually become hair. At this age, the babies can start moving around inside the uterus. They might move their arms and legs or simply flutter their hands and feet. From 10 weeks, you can hear hearts beating separately. Also at this stage, the bones begin to harden into shape, and the brain starts producing hormones that control growth and development of other organs. By 11 weeks, most babies can roll over from back to front.