Approximately 16% of twin gestations have discordance of at least 20%. We identified 14 risk factors for divergent growth that can be categorized as maternal, fetal, or placental. Determination of chorionicity and serial ultrasound evaluation with a high index of suspicion for divergent growth is required for the diagnosis and stratification of risk. The highest reported likelihood ratio for detection of discordance was 5.9 during the first trimester examination and 6.0 for the second trimester. Although our ability to identify discordant twins is limited, once suspected and at viable gestational age, these pregnancies should have antepartum testing. Discordant growth alone is not an indication for preterm birth. Although there are multiple publications on the increased morbidity and mortality rates with discordant growth, there is a paucity of reports on how to manage them optimally and deliver them in a timely manner.
Unique to multiple gestations, discordance is the difference in the weights of the fetuses. According to the American College of Obstetricians and Gynecologists (ACOG) practice bulletin on multiple gestation, discordant growth is associated with increased likelihood of anomalies, intrauterine growth restriction (IUGR), preterm birth, infection of 1 fetus, stillbirth, umbilical arterial pH <7.10, admission to neonatal intensive care unit, respiratory distress, and death within 1 week of birth. Despite the known association with a multitude of adverse outcomes, what is debated about discordant twins are the following factors that decrease or predispose to discordant growth: the ability to identify abnormal growth, the threshold of discordance that significantly increases the perinatal complication rate, the comorbidities that alter the likelihood of poor outcome, and how to manage divergent growth.
The purpose of this review article was to summarize the literature on discordant growth among nonanomalous twins. We will discuss the various definitions, risk factors, and evaluation and management strategies. Because of the breadth of the subject matter, we will not focus on monoamniotic twins, twin-twin transfusion syndrome, anomalous fetuses, death of a twin, or suboptimal growth among twins.
Definition
Discordance is defined with the larger twin as the standard of growth and is calculated by the following equation: (larger estimated or actual weight – smaller estimated or actual weight)/larger estimate or actual weight). While acknowledging the lack of consensus on the precise threshold of discordance that is linked with complications, ACOG considers a 15-25% difference in actual weight among twins to be discordant. The consensus statement by the Society of Obstetricians Gynecologists of Canada specifies that discordance is a difference of abdominal circumference (AC) of 20 mm or estimated fetal weight (EFW) difference of 20%. the Society of Obstetricians Gynecologists of Canada recommends that the EFW be derived from biparietal diameter with AC or a combination of AC and femur length.
Prevalence and detection of discordant growth
A summary of 31 publications with >1.1 million twins indicates that the likelihood of discordance of ≥20% is 16% (180,302/1,130,505 twin pregnancies; range, 14–41%; Table 1 ). Eight publications provided evidence of discordance of at least 30%; discordance has occurred in 5% of twins (42,373/854,331 twin pregnancies; range, 3–10%. The rate of discordance, however, varied among publications with <1000 vs 1000-9999 vs ≥10,000 cohorts ( Figure 1 ; P < .0001 for both comparisons). Discordance of at least 20% was significantly higher in 15 publications from foreign countries (17%; 11,369/65,997 twin pregnancies ) than in 16 reports from the United Sates (16%; 168,933/1,064,790 twin pregnancies; odds ratio [OR], 1.10; 95% confidence interval [CI], 1.08–1.12 ). Even among publications with <1000 cohorts, discordance of >20% occurred significantly more commonly in other countries (19%; 520/2,712 twin pregnancies ) than in the United States (16%; 225/1,445 twin pregnancies; OR, 1.28; 95% CI, 1.08–1.52 ). In 2006, there were 137,085 twin pairs born in the United States; if 16% were discordant, we estimate that there are approximately 22,000 discordant twin pairs born per year.
| Study | Year | Country | Study period | Twins, n | Discordance ≥20%, n | Discordance ≥20%, % |
|---|---|---|---|---|---|---|
| Watson et al | 1991 | USA | No mention | 94 | 21 | 22 |
| Eberle et al | 1993 | USA | 1986-1992 | 147 | 36 | 24 |
| Jensen and Jenssen | 1995 | Norway | 1990-1993 | 73 | 14 | 19 |
| Cheung et al | 1995 | Canada | 1989-1992 | 122 | 28 | 23 |
| Blickstein et al | 1996 | Israel | No mention | 90 | 20 | 22 |
| Yalçin et al | 1998 | Turkey | 1994-1995 | 357 | 115 | 32 |
| Hollier et al | 1999 | USA | 1988-1996 | 1370 | 194 | 14 |
| Grobman and Parilla | 1999 | USA | 1992-1988 | 44 | 18 | 41 |
| Foley et al | 2000 | USA | No mention | 500 | 62 | 12 |
| Demissie et al | 2002 | USA | 1995-1997 | 148,577 | 24,190 | 16 |
| Kalish et al | 2003 | USA | 2000-2002 | 130 | 16 | 12 |
| Branum and Schoendorf | 2003 | USA | 1995-1997 | 128,163 | 19,253 | 15 |
| Sannoh et al | 2003 | USA | 1995-1997 | 294,568 | 47,796 | 16 |
| Smiljan Severinski et al | 2004 | Croatia | 1993-2001 | 351 | 53 | 15 |
| Amaru et al | 2004 | USA | 1992-2001 | 1318 | 208 | 16 |
| Chauhan et al | 2004 | USA | No mention | 126 | 24 | 19 |
| Usta et al | 2005 | Lebanon | 1984-2000 | 679 | 81 | 12 |
| Tan et al | 2005 | USA | 1995-1997 | 147,262 | 23,071 | 16 |
| Kontopoulos et al | 2005 | USA | 1995-1998 | 340,446 | 53,584 | 16 |
| Wen et al | 2005 | Canada | 1986-1997 | 59,034 | 10,092 | 17 |
| Armson et al | 2006 | Canada | 1988-2002 | 1542 | 211 | 14 |
| Chang et al | 2006 | Taiwan | 1991-2002 | 1257 | 195 | 16 |
| Canpolat et al | 2006 | Turkey | 2000-2004 | 266 | 54 | 20 |
| Pongpanich and Borriboonhirunsarn | 2006 | Thailand | 2003-2004 | 150 | 35 | 23 |
| Tai and Grobman | 2007 | USA | 2000-2006 | 169 | 24 | 14 |
| Belogolovkin et al | 2007 | USA | 2000-2005 | 279 | 42 | 15 |
| Appleton et al | 2007 | Portugal | 1989-2002 | 230 | 54 | 23 |
| Hack et al | 2008 | Netherlands | 1995-2004 | 1305 | 351 | 27 |
| Banks et al | 2008 | United Kingdom | 2002-2004 | 108 | 26 | 24 |
| Nawab et al | 2008 | USA | 2001-2004 | 1597 | 394 | 25 |
| Alam Machado Rde et al | 2009 | Brazil | 1998-2004 | 151 | 40 | 26 |
| T otal | 1,130,505 | 180,302 | 16 |
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