Monochorionic twins
（重定向自Monochorionic）
![Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides.](/Images/godic/202501/28/Placentation.svg5553.png")
![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.](/Images/godic/202501/28/Monoamniotic_twins_at_15_weeks5553.jpg")
![λ Sign in 8 week pregnancy](/Images/godic/202501/28/Ultrasound_Scan_ND_0345553.jpg")
Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. 75% of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic. If the placenta divides, this takes place after the third day after fertilization.