Definition of Intertwin Birth Weight Discordance

Breathnach, Fionnuala M. MD; McAuliffe, Fionnuala M. MD; Geary, Michael MD; Daly, Sean MD; Higgins, John R. MD; Dornan, James MD; Morrison, John J. MD; Burke, Gerard FRCOG; Higgins, Shane FRCOG; Dicker, Patrick PhD; Manning, Fiona PhD; Mahony, Rhona MD; Malone, Fergal D. MD; for the Perinatal Ireland Research Consortium

Obstetrics & Gynecology:
doi: 10.1097/AOG.0b013e31821fd208
Original Research

OBJECTIVE: To establish the level of birth weight discordance at which perinatal morbidity increases in monochorionic and dichorionic twin pregnancy.

METHODS: This prospective multicenter cohort study included 1,028 unselected twin pairs recruited over a 2-year period. Participants underwent two weekly ultrasonographic surveillance from 24 weeks of gestation with surveillance of monochorionic twins two-weekly from 16 weeks. Analysis using Cox proportional hazards compared a composite measure of perinatal morbidity (including any of the following: mortality, respiratory distress syndrome, hypoxic–ischemic encephalopathy, periventricular leukomalacia, necrotizing enterocolitis, or sepsis) at different degrees of birth weight discordance with adjustment for chorionicity, gestational age, twin–twin transfusion syndrome, birth order, gender, and growth restriction.

RESULTS: Perinatal outcome data were recorded for 977 patients (100%) who continued the study with both fetuses alive beyond 24 weeks, including 14 cases of twin–twin transfusion syndrome. Adjusting for gestation at delivery, twin order, gender, and growth restriction, perinatal mortality, individual morbidity, and composite perinatal morbidity were all seen to increase with birth weight discordance exceeding 18% for dichorionic pairs (hazard ratio 2.2, 95% confidence interval [CI] 1.6–2.9, P<.001) and 18% for monochorionic twins without twin–twin transfusion syndrome (hazard ratio 2.6, 95% CI 1.6–4.3, P<.001). A minimum twofold increase in risk of perinatal morbidity persisted even when both twin birth weights were appropriate for gestational age.

CONCLUSION: The threshold for birth weight discordance established by this prospective study is 18% both for dichorionic twin pairs and for monochorionic twins without twin–twin transfusion syndrome. This threshold is considerably lower than that defined by many retrospective series as pathologic. We suggest that an anticipated difference of 18% in birth weight should prompt more intensive fetal monitoring.


In Brief

The threshold for birth weight discordance established by this prospective study is 18 for both dichorionic twin pairs and for monochorionic twins without twin&amp;#x2013;twin transfusion syndrome.

Author Information

From the Royal College of Surgeons in Ireland, the UCD School of Medicine and Medical Science, University College Dublin, the National Maternity Hospital, the Rotunda Hospital, Coombe Women and Infants' University Hospital, Dublin, Ireland; Anu Research Centre, University College Cork, Cork, Ireland; Royal Victoria Maternity Hospital, Belfast, Northern Ireland, United Kingdom; the National University of Ireland, Galway, Ireland; Mid-Western Regional Maternity Hospital, Limerick, Ireland; and Our Lady of Lourdes Hospital, Drogheda, Ireland.

Supported by grant from Health Research Board of Ireland (Grant Code IMA/2005/3).

The Perinatal Ireland Research Consortium thanks the following research sonographers and research staff, who were responsible for recruitment to the study, performed all serial ultrasonographic examinations on all study participants, and were also responsible for uploading ultrasound, perinatal and neonatal data onto a central consolidated web-based database: Ms. Cecilia Mulcahy, Ms. Fiona Cody, Ms. Hilda O'Keefe, Ms. Phyl Gargan, Ms. Emma Doolin, Ms. Marion Cunningham, Dr. Richard Horgan, Dr. Mary Higgins, Ms. Annette Burke, Ms. Deborah McCartan, Dr. Cassie Staehr, and Ms. Bernadette McPolin.

Presented at the Society for Maternal-Fetal Medicine 31st Annual Pregnancy Meeting, February 7–12, 2011, San Francisco, California.

Corresponding author: Fionnuala M. Breathnach, MD, Royal College of Surgeons in Ireland, Rotunda Hospital, Parnell Square, Dublin 1; e-mail:

Financial Disclosure The authors did not report any potential conflicts of interest.

© 2011 The American College of Obstetricians and Gynecologists