Imaging Considerations Determining Chorion and Amnion Status Recently, noninvasive prenatal determination of twin zygosity by maternal plasma DNA sequencing is reported as feasible. .
However, for the foreseeable future ultrasound will remain the primary determinant of zygosity and amnion and chorion status in defining the type of multiple gestations. Both transvaginal ultrasound and increasingly improved imaging modalities make this possible.
Perform transvaginal ultrasound at 6 to 7 weeks at the earliest. In dichorionic diamniotic placentas, the chorion appears as separate rings and there is a thick dividing membrane and an embryo in each sac. In diamniotic monochorionic placentas, there is one chorionic ring, a dividing membrane and an embryo in each sac. In monoamniotic monochorionic pregnancies, there is no dividing membrane and 2 embryos in a single sac.
Between 9 and 12 weeks gestation, the dividing membrane should be clearly visible on transvaginal ultrasound. A thick dividing membrane suggests a diamniotic placenta and a thin dividing membrane suggests a monochorionic placenta. No dividing membrane suggests monoamniotic monochorionic twins. Measure the CRL for each twin. The median CRL discrepancy was 11.9% between twins with fetal growth restriction while the median CRL discrepancy was 3.8% for the TTTS group and 3.5% for the control group. 
Count the number of yolk sacs.
The yolk sac and the amnion develop temporally before 6 weeks gestation but the sequence of development is not clear. 
In MC twin pregnancies, the number of yolk sacs determined between 6 and 9.5 weeks often correlates with the amnion status. (1 yolk sac = 1 amnion) . That is, if 2 yolk sacs are seen, a diamniotic monochorionic pregnancy can be assumed while the presence of 1 yolk sac was assumed to represent a monochorionic monoamniotic twin gestation. Recently, that observation has been challenged since 2 yolk sacs have associated with monoamniotic monochorionic pregnancies.  Others reported similar observations.  In addition, a single yolk sac occurs in diamniotic monochorionic pregnancies in 3 of 20 or 15% of cases. 
Conclusions: Among monochorionic placentas observed during the early first trimester 2 yolk sacs are usually but not always associated with the presence of diamniotic monochorionic gestation. The presence of a single yolk sac may also be associated with a diamniotic monochorionic pregnancy while the presence of 2 yolk sacs does not exclude the possibility of monoamniotic monochorionic pregnancy.
3. Amnion status can be determined by counting the number of embryonic heart beats within each individual gestation sac. Two gestational sacs and 2 separate heart beats would suggest diamniotic status.
4. Measure the nuchal fold (NT) for each twin.
The nuchal fold measurement above the 95th percentile is correlated with fetal aneuploidy and certain malformations. Among a large series of monochorionic twins (N=242), median inter-twin NT discrepancies were not different between those with who developed TTTS and controls. 
Mid-trimester and third Trimester 1. Determine the number of placentas. If the placentas are physically separate (i.e., posterior and anterior wall of the uterus), diamniotic dichorionic placentas are most likely.
2. Determine if the placentas are fused. If the placentas are fused, any type of
gestations are possible, and then assess membrane characteristics.
3. If the membrane is thick, diamniotic dichorionic placentas are most likely.
If the membrane is wispy and thin, diamniotic monochorionic placentas are most likely.
If there is no dividing membrane, monoamniotic monochorionic placentas are most likely.
4. Twin peak sign is a portion of tissue extending into the base of inter-twin
membrane and is associated with diamniotic dichorionic gestation. 
5. If in doubt, count the number of layers within the dividing membrane. If greater than 2, diamniotic dichorionic placentas are most likely. 
6. Determine gender. If the genders are different, diamniotic dichorionic placentas are most likely.
Accuracy in Ultrasound Determination of Chorion and Amnion Status For the determination of chorion status and amnion status in twin pregnancies, transvaginal ultrasound at between 7 and 9 weeks gestation is comparable in accuracy (97%) to an ultrasound scan performed at 11 to 14 weeks gestation.  PMID 18677702 Sonographers and perinatologist scanning the same patients have a rate of discordant results of less than 1% among diamniotic dichorionic pregnancies and a rate of discordant results of 5.5% among diamniotic monochorionic pregnancies. [ 10] PMID 16170849 Based upon placental pathology, correct prediction of chorion status was made in all but 2 of 82 patients scanned at between 5.5 and 26 weeks gestation.  PMID 11665300
1. Qu JZ, Leung TY, Jiang P, Liao GJ, Cheng YK, Sun H, Chiu RW, Chan KC, Lo YM.Clin Chem. 2012 Oct 31.Noninvasive Prenatal Determination of Twin Zygosity by Maternal Plasma DNA Analysis.
2. Memmo A, Dias T, Mahsud-Dornan S, Papageorghiou AT, Bhide A, Thilaganathan B. Prediction of selective fetal growth restriction and twin-to-twin transfusion
syndrome in monochorionic twins..BJOG. 2012 Mar;119(4):417-21. PMID: 22324917
3. Murakoshi T, Ishii K, Matsushita M, Shinno T, Naruse H, Torii Y.Ultrasound Obstet Gynecol. 2010 Sep;36(3):384-6. Monochorionic monoamniotic twin pregnancies with two yolk sacs may not be a rare
finding: a report of two cases
4. Bromley B, Benacerraf B.Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins. JUM June 1, 1995 vol. 14 no. 6 415-419 PMID: 7658507
5. Corbett, SL, Shmorgun, D. Yolk sac number does not predict reliably amnionicity in monochorionic twin pregnancies: a case of a monochorionic monoamniotic twin pregnancy with two distinct yolk sacs on early first-trimester ultrasound ULTRASOUND IN OBSTETRICS & GYNECOLOGY Volume: 39 Issue: 5 Pages: 607-608 MAY 2012 PMID: 22461300
6. Shen O, Samueloff A, Beller U, Rabinowitz R.. Ultrasound Obstet Gynecol. 2006 Jan;27(1):53-5.
Number of yolk sacs does not predict amnionicity in early first-trimester monochorionic multiple gestations. PMID: 16276551
7. Finberg HJ.
The "twin peak" sign: reliable evidence of dichorionic twinning.. J Ultrasound Med. 1992 Nov;11(11):571-7. PMID: 1433464 [PubMed - indexed for MEDLINE]
8. D'Alton ME, Dudley DK.. A The ultrasonographic prediction of chorionicity in twin gestation. Am J Obstet Gynecol. 1989 Mar;160(3):557-61. PMID: 2648832 [PubMed - indexed for MEDLINE]
the 11-14-week scan. There was one monochorionic-monoamniotic (MCMA) pregnancy
diagnosed at 7-9 weeks that was subsequently found to be MCDA at the 11-14-week
CONCLUSION: Transvaginal ultrasound examination at 7-9 weeks' gestation shows
very high agreement with the 11-14-week scan in the diagnosis of chorionicity and
amnionicity in twin pregnancies, suggesting that it provides a similar level of
accuracy. Accuracy may be higher for DC twins than MC twins, which may relate to
the gestational age at which the sonographic appearance of the amniotic sac
PMID: 18677702 [PubMed - indexed for MEDLINE]
10. Prenat Diagn. 2005 Sep;25(9):835-8.
Scanning for chorionicity: comparison between sonographers and perinatologists.
Weisz B, Pandya P, Dave R, Jauniaux E.
Department of Obstetrics and Gynaecology, University College London, London, UK.
RESULTS: Chorionicity was determined in 172 twin cases by the two different
departments. The overall rate of concordant chorionicity determination between
both units was 90.1%. The rate of discordant results in dichorionic pregnancies
was extremely small, 1 in 119 pregnancies (0.8%). The rate of discordant results
for monochorionic diamniotic pregnancies was 5.5%. Monoamniotic pregnancies were
over-diagnosed by the US technicians.
DISCUSSION: These results demonstrate that DC/DA chorionicity is accurately
determined by sonographers at less than 14 weeks. In our opinion, it is both
efficient and safe to rely on the diagnosis of the sonographers in DC/DA
pregnancies in early pregnancy. In such pregnancies, a decision can be made
either not to refer these patients for further evaluation of chorionicity by the
fetal medicine team or to postpone the referral to after 14 weeks.
2005 John Wiley & Sons, Ltd.
PMID: 16170849 [PubMed - indexed for MEDLINE]
11. 1. Twin Res. 2001 Aug;4(4):223-6.
Ultrasound determination of chorionicity in twin pregnancy: accuracy and operator
Devlieger RG, Demeyere T, Deprest JA, Van Schoubroeck D, Witters I, Timmerman D,
Department of Obstetrics and Gynecology, University Hospital Gasthuisberg,