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Findings: (Sonography) Inseparable fetal bodies and contours. No change in relative position at fetuses. Single umbilical cord with more than 3 vessels. Most conjoined twins are fused ventrally and face each other leading to the backward flexion of the cervical spine.
Shared heart or other organs.
Polyhydramnios occurs in 50% of conjoined twin pregnancies compared to 10% of normal twin and 2% of normal singleton pregnancies.
Discordant presentation does not exclude the diagnosis of conjoined twins. In fact, cephalic - breech presentation is the most common presentation for omphalopagus twins.
Diagnosis:Conjoined twins
Discussion:
Incidence:
Thoracopagus - shared thorax. 90% have a shared heart. 75% have extensively conjoined hearts that preclude surgical separation.
Omphalopagus - shared abdomen. This does not mean the twins have an omphalocele. However, the incidence of omphalocele is increased. Conjoined livers are very common and there is an increased incidence of congenital heart disease not obviously linked to the conjoined abdomen.
Thoraco-omphalopagus - one of the most common types.
Ileopagus - connected at the iliac bone. When the twins are extensively connected then the duplicated part is named. For example, dicephalus refers to two heads with one body.
Embryology: Only monozygotic twins can be conjoined. Monozygotic or identical twins account for 30% of all twins.
Four days after fertilization the trophoblast (chorion) differentiates. If the split occurs before this time the monozygotic twins will implant as separate blastocysts each with their own chorion and amnion. 25% of monozygotic twins are dichorionic. All dichorionic twins are diamniotic.
Eight days after fertilization the amnion differentiates. If the split occurs between the 4th and 8th days, then the twins will share the same chorion but have separate amnions. Monochorionic diamniotic is the most common form at monozygotic twins, accounting for 75% of monozygotic twins.
If a split occurs after the 8th day and before the 13th day, then twins will share the same chorion and amnion. This is a very rare condition and accounts for 1-2% of monozygotic twins.
The embryonic disk starts to differentiate on the 13th day. If the split occurs after day 13, then the twins will share body parts in addition to sharing their chorion and amnion.
References:
1. Barth RA, Filly RA, Goldberg JD, et al. Conjoined Twins Prenatal Diagnosis
and Assessment of Associated Malformations. Radiology 1990;177:201-207.
2. Filly RA, Goldstein, RB and Callen PW. Monochorionic Twinning: Sonographic
Assessment. AJR 154:459-469, March 1990.
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