1. UOG Journal Club: August 2011
Systemic labeling of twin pregnancies on ultrasound
T Dias, S Ladd, S Mahsud-Dornan, A Bhide, A T Papageorghiou and B Thilaganathan
Volume 38, Issue 2, Date: August 2011, pages 130–133
Journal Club slides prepared by Dr Asma Khalil
(UOG Editor for Trainees)

2. Multiple pregnancy 1–2% of all conceptions 20 22 24 26 28 30 32 34
1994
1996
1998
2000
2002
2004
2006
2008
2010
(per 1000 total births)
USA twin birth rate 1995–2008
1–2% of all conceptions
National Vital Statistics Reports 2010

3. Twin pregnancies are at increased risk of:
Multiple pregnancy
Twin pregnancies are at increased risk of:
Discordant
anomaly
Aneuploidy
Growth
restriction
Accurate labeling is essential especially if invasive prenatal diagnosis or therapy is required
Sebire NJ et al., BJOG 1997
Barigye O et al., PLoS Med 2005
Bhide A et al., UOG 2010
Centre for Maternal and Child Enquiries (CMACE) Perinatal Mortality 2008:UK

4. Current twin labeling methods based on proximity to cervix
Presenting twin
Twin 1 on left
Presenting sac
Twin 1 on right
Evidence: none

5. Systemic labeling of twin pregnancies on ultrasound
Dias et al., UOG 2011
Retrospective; 416 twin pregnancies at 11–14 weeks; 2000–2010
Objectives
1. Describe a standard method of twin labeling in the first trimester of pregnancy
2. Assess the reliability of this technique in predicting the presenting twin in subsequent scans
3. Assess the reliability of this technique in predicting the birth order at delivery

6. Methodology 11–14 weeks Subsequent scans: Birth order: Lateral
The cervical canal was identified
Relative orientation of the twins
Lateral
Vertical
Subsequent scans:
Which twin (left or right, top or bottom) was the presenting twin?
Birth order:
Sex and presenting order on the final scan prior to delivery was compared to the sex and birth order at delivery
(N.B. neonatal convention dictates that first born is always Twin 1)
Lateral
(left/right)
Vertical
(top/bottom)

7. Results Subsequent scans: Birth order: Vertical (top/bottom) 38 (9%)0% 9%
Lateral (right/left)
378 (91%)
Birth order:
Subsequent scans:
Changed by last scan
11–14 weeks
Cesarean delivery 74
20%
5.9%
Vaginal delivery 34
Change in twin order
Number
Total
108
15.7%

8. Lateral and vertical orientation unchanged throughout pregnancy
10%
Presenting twin determined by proximity to cervix changes in 10% of left/right orientated pregnancy

9. Peripartum (birth order) switch6%
A peripartum switch in vaginal birth order may occur if
Twin 2 delivers through a fold of the intertwin membrane.

10. Twins labeling – options and limitations Gestational sac position
Placental position
Changes with advancing gestation
Not in monochorionic or fused dichorionic twin pregnancy
Fetal sex
Fetal sexing unreliable in early pregnancy
Precluded in same-sex twin pregnancy
Gestational sac position
and orientation
The position of the gestational sac in relation to the cervix remains constant
The base of the intertwin membrane is immobile
Fetal position
The position of either fetus relative to the cervix changes in pregnancy
Fetuses are free to move independently of each other

11. Systemic labeling of twin pregnancies on ultrasound
Dias et al., UOG 2011
Conclusion
Antenatal labeling of twins according to laterality or vertical orientation is reliable
The use of orientation for antenatal labeling of twins rather than assignment of a number based on proximity to the cervix precludes misconceptions regarding which twin will be born first