Anatomy of Monochorionic diamniotic twin

Monochorionic diamniotic twin pregnancy is a type of twin pregnancy where each twin has its own amniotic sacs
but both share one chorion

1. Monochorionic amniotic twin pregnancy

2. Teratology

3. Monochorionic diamniotic twin pregnancy is a type of twin pregnancy where each twin has its own amniotic sacs but both share one chorion

4. Pathophysiology: An MCDA twin pregnancy result from the fertilization of one ovum by one sperm. The resulting zygote forms single blastocyst which contain two the inner cell masses (embryoblast ) but single outer cell mass .

5. Pathophysiology: Single outer cell mass forms a single chorionic sac and single placenta but two  inner cell masses produce two yolk sacs  and two  amniotic sacs .

6. Types of Monozygotic Twins Related to Time of Division After Ovum Fertilization Chorion Amnion Time to Division (Days) Frequency Dichorionic Diamniotic 0-3 25% Monochorionic Diamniotic 4-8 75% Monochorionic Monoamniotic 9-12 ~1% Monochorionic Monoamniotic 13-15 Rare conjoined twins

7. Radiographic features

8. 1 st trimester early

9. 1 st trimester dichorionic diamniotic twin Dichorionic twin pregnancy at 5 weeks 3 days. Two round sonolucent sacs with a brightly echogenic rim are clearly visible in the thick decidua.

10. 1 st trimester monochorionic diamniotic twin At 6 postmenstrual weeks a single chorionic sac is seen containing two yolk sacs: the diagnosis of monochorionic twin pregnancy can be done; it is not possible yet to diagnose amnioticity ( monochorionic or diamniotic )

11. 1 st trimester Monochorionic twin pregnancy At 6 postmenstrual weeks a single chorionic sac is seen containing two yolk sacs: the diagnosis of monochorionic twin pregnancy can be done; it is not possible yet to diagnose amnioticity . Dichorionic twin pregnancy at 5 weeks 3 days. Two round sonolucent sacs with a brightly echogenic rim are clearly visible in the thick decidua.

12. Ultrasound First trimester early shows a twin pregnancy with a single gestational sac, and almost always two separate yolk sacs  (differentiating from an  MCMA  pregnancy)

13. Ultrasound First trimester late T-sign  of the intertwin membrane a thin  inter-twin membrane  may be seen but appears very thin without intervening chorion (often taken as <2 mm): differentiating from a  DCDA  pregnancy (although this assessment becomes increasingly difficult with the progression of pregnancy )

14. 1 st trimester late

15. Monochorionic diamniotic twin pregnancy Dichorionic diamniotic twin pregnancy

16. Second and third trimesters Findings noted on a second trimester scan include: the number of placental masses, thickness of the membrane, and the presence/absence of the twin-peak sign are still viable options for determining chorionicity fetal sex : almost always the same sex Negative findings: absent  twin peak sign : differentiating from a  DCDA  pregnancy

17. Complications Potential complications that can occur with this type of pregnancy include: problems related to abnormal placental vascular anastomoses twin to twin transfusion syndrome: can occur in ~15  1  - 30  4  % of MCDA pregnancies twin embolization syndrome twin reversed arterial perfusion sequence :

18. Complications demise of one twin: often associated with some adverse outcome to the other twin

19. Complications placental insertion related problems increased incidence of  velamentous cord insertion  increased incidence of marginal cord insertion ( c.f singleton pregnancy)

20. Vascular anastomoses include 3 types: arterioarterial (AA), venovenous ( VV ) and arteriovenous (AV) anastomoses.

21. arterioarterial (AA ), venovenous ( VV ) anastomosis are superficial with bidirectional blood flow and directly linking the arteries and veins of two umbilical cords, while AV anastomoses form at a deep capillary level within shared cotyledons and allow only unidirectional blood flow. Monochorionic Diamniotic Twin Pregnancies by Video of Tara A Morgan, MD

22. Complications placental insertion related problems

23. Thank you