Abdominal ultrasonography of<unk>  at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right. Ultrasound is the only way to detect<unk> -monochorionic twins before birth. It can show the lack of a membrane between the twins after a couple of weeks' gestation, when the membrane would be visible if present. Further ultrasounds with high resolution doppler imaging and non-stress tests help to assess the situation and identify potential cord problems. There is a correlation between having a single yolk sac and having a single amniotic sac. However, it is difficult to detect the number of yolk sacs, because the yolk sac disappears during embryogenesis. Cord entanglement and compression generally progress slowly, allowing parents and medical caregivers to make decisions carefully.
Monoamniotic twins
Monochorionic twins
Locked twins
Conjoined twins
Parasitic twin
Craniopagus twins
Vanishing twin
Vestigial twin