Imaging Considerations
Determining Chorion and Amnion Status
Recently, noninvasive prenatal determination of twin zygosity by maternal plasma DNA sequencing is
reported as feasible. [1].
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.
First trimester
1.
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. [2]
2. 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. [3]
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) [4]. 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. [3] Others reported similar observations. [5] In addition, a single
yolk sac occurs in diamniotic monochorionic pregnancies in 3 of 20 or 15% of cases. [6]
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. [2]
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. [7]
5.
If in doubt, count the number of layers within the dividing membrane. If greater than 2,
diamniotic dichorionic placentas are most likely. [8]
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. [9] 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. [11] PMID 11665300
References
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.
PMID: 23115054
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
PMID: 20533442
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]
9. Ultrasound Obstet Gynecol. 2008 Oct;32(5):618-21. doi: 10.1002/uog.6133.
Reliability of transvaginal ultrasonography at 7-9 weeks' gestation in the
determination of chorionicity and amnionicity in twin pregnancies.
Bora SA, Papageorghiou AT, Bottomley C, Kirk E, Bourne T.
Early Pregnancy and Gynaecological Ultrasound Unit, St George's, University of
London, London, UK. SBora@doctors.org.uk
OBJECTIVES: To assess the agreement in the diagnosis of chorionicity and
amnionicity between transvaginal ultrasound examination at 7-9 weeks' gestation
and at the 11-14-week transabdominal scan.
METHODS: This was a retrospective analysis of prospectively collected data of
twin pregnancies over a 3-year period. Chorionicity and amnionicity were assessed
in women with viable twin pregnancies who underwent a transvaginal ultrasound
scan at between 7 and 9 weeks' gestation. These findings were compared with the
diagnoses of chorionicity and amnionicity at the subsequent 11-14-week scan in
these women.
RESULTS: Chorionicity and amnionicity were documented in 67 viable twin
pregnancies at both 7-9 and 11-14 weeks' gestation. There was agreement in the
chorionicity and amnionicity reported at each of the two scans in 65 out of 67
(97%) cases. Of the dichorionic-diamniotic (DCDA) pregnancies reported at 7-9
weeks, 53 out of 54 (98%) were confirmed at the 11-14-week scan and one (2%) was
found to be monochorionic-diamniotic (MCDA). However, at birth these twins were
of different sex, confirming DCDA twins as initially diagnosed at 7-9 weeks. Of
the 12 pregnancies diagnosed as MCDA at 7-9 weeks, all were found to be MCDA at
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
scan.
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
develops.
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.
b.weisz@ucl.ac.uk
OBJECTIVE: In most prenatal settings, twin pregnancies are initially evaluated by
sonographers. Pregnancies diagnosed as monochorionic are subsequently referred to
perinatologists or specialists in fetal medicine for the confirmation of
chorionicity. In order to assess this screening strategy, we have compared the
diagnosis of chorionicity made by the sonographers in the ultrasound department
with the diagnosis done in the fetal medicine unit.
METHODS: A cohort of women presenting with twin pregnancy and booked for prenatal
care at University College London Hospitals over a 4-year period were
investigated prospectively. All women were scanned at their initial visit at
11-14 weeks in the ultrasound department (US), and were subsequently referred to
the Fetal Medicine Unit (FMU) for a second ultrasound evaluation. Ultrasound data
were compared and diagnosis of chorionicity was confirmed by examination of the
inter-twin membranes after delivery.
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
experience.
Devlieger RG, Demeyere T, Deprest JA, Van Schoubroeck D, Witters I, Timmerman D,
Hanssens M.
Department of Obstetrics and Gynecology, University Hospital Gasthuisberg,
Katholieke Universiteit Leuven, Belgium. roland.devlieger@uz.kuleuven.ac.be
To evaluate the accuracy of ultrasound in early detection of chorionicity and
amnionicity in twin pregnancies and evaluate the role of operator-experience, a
prospective follow up of 87 consecutive twin pregnancies was performed.
Chorionicity and amnionicity were determined by 17 different observers, divided
in 2 groups on the basis of their experience, using a composite of US markers at
the first US examination confirming a viable intrauterine twin pregnancy.
Accuracy of prediction was determined for both groups by comparison with
postnatal pathology. From the 87 pregnancies examined at 10.1 (minimum 5.5 weeks,
maximum 26.0) weeks of gestation, pathology was available in 82 cases and a
correct prediction of chorionicity was made in all but two cases, both being
diagnosed as monochorionic while dichorionic by observers belonging the more
experienced group. No monochorionic pregnancy was missed and amnionicity was
correctly diagnosed in all cases, including two monoamniotic twins. We conclude
that use of a composite of ultrasound markers for early detection of chorionicity
and amnionicity is reliable in a clinical setting where physicians with a
variable degree of experience perform ultrasound examination.
PMID: 11665300 [PubMed - indexed for MEDLINE]