Embryology · Fetal period, placenta and twins
Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion !
- Topic
Twins come from two eggs or from one embryo that splits. When it splits decides whether they share a placenta and a sac, and that sets the risk.
Updated 2026-11-01
See it in 3D: Body of uterus
What it is
Dizygotic twins come from two eggs fertilized by two sperm and always have two chorions and two amnions. Monozygotic twins come from one zygote that splits, and the timing of the split decides how many placentas and sacs they share. Chorionicity, best defined on first-trimester ultrasound, sets the risk: only monochorionic twins get twin–twin transfusion.
Key facts
- In dizygotic twins the two placentas can fuse, but they remain dichorionic.
- A dichorionic pair can be monozygotic if the split came in the first three days; different sexes, by contrast, confirm dizygosity.
- Monoamniotic twins risk cord entanglement.
- Conjoined twins are named by where they join: thoracopagus (chest), omphalopagus, craniopagus, pygopagus.
Timing of the split and the membranes
| Timing of split | Stage | Chorion and amnion |
|---|---|---|
| Two eggs (dizygotic) | Two zygotes | Dichorionic diamniotic |
| Days 0–3 | Two-cell stage to morula | Dichorionic diamniotic |
| Days 4–8 | Blastocyst: inner cell mass splits | Monochorionic diamniotic |
| Days 8–12 | Bilaminar disc, amnion already formed | Monochorionic monoamniotic |
| After day 13 | Primitive streak already forming | Conjoined twins, monochorionic monoamniotic |
Clinical notes
- Twin–twin transfusion: the donor is anaemic, small and oligohydramniotic; the recipient has polyhydramnios, polycythaemia and heart failure.
- Vanishing twin: death and resorption of one twin in the first trimester.
- Twin pregnancy raises the risk of preterm birth, low birth weight, preeclampsia and gestational diabetes.
- Severe twin–twin transfusion is treated with fetoscopic laser coagulation of the anastomoses.
Test yourself
Think of the answer, then tap the question to see it.
What type of twins results if the embryo splits between days 4 and 8?
Monochorionic diamniotic twins: they share a placenta and chorion but each has its own amnion. This is the commonest type of monozygotic twins.
Which membranes do dizygotic twins always have?
Two chorions and two amnions (dichorionic diamniotic), even when their placentas fuse.
Why does twin–twin transfusion occur only in monochorionic twins?
Because it needs a shared placenta with vascular anastomoses between the two fetal circulations.
What does the lambda sign on twin ultrasound indicate?
That the pregnancy is dichorionic: chorionic tissue extends between the two membranes at their base.
What results from a split after day 13?
Conjoined twins, because the split is incomplete once the primitive streak has formed.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Week 1: cleavage, morula, blastocyst and implantation
- Week 2: the bilaminar disc ('the week of twos')
- Fetal membranes: amnion, chorion, yolk sac and allantois
- Placenta: structure, circulation and functions
- Amniotic fluid: polyhydramnios and oligohydramnios
- Fetal growth and growth restriction
- Fertilization: capacitation, acrosome reaction and zona reaction
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Twin-to-twin transfusion syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Twins, Triplets, Multiple Births: MedlinePlus (MedlinePlus, U.S. National Library of Medicine, 2024); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.