Monochorionic twins

Monoamniotic twins occur when the split takes place after the ninth day after fertilization.

At ultrasound at a gestational age of 16–20 weeks, the "lambda sign" is indicative of dichorionicity but its absence does not exclude it.

This can cause disproportionate blood supply, resulting in twin-to-twin transfusion syndrome (TTTS) in 20%[1] of MoDi pregnancies.

The 80% of MoDi pregnancies without TTTS still have high rates of birth weight discordance, fetal growth restriction, prematurity and resultant cesarean section deliveries.

[2] In the case of monoamniotic twins the risk of complications is substantially higher because of additional potential umbilical cord entanglement and compression.

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (identical) twins as a result of when the fertilized egg divides.
Abdominal ultrasonography of monoamniotic twins 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.
λ Sign in 8 week pregnancy