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A Difference in First Trimester Crown-Rump Length in Twins is Related to Birth-Weight Discordance

Background. Discordant twin growth is believed by many to be a direct result of the process of twinning and of the inability of the uterine environment to provide for the increased demands of multiple fetuses.1Discordant fetal growth has been reported to complicate 15-29% of twin gestations, with an associated increase in perinatal morbidity and mortality, especially for the smaller twin.2,3.

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A Difference in First Trimester Crown-Rump Length in Twins is Related to Birth-Weight Discordance

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    1. A Difference in First Trimester Crown-Rump Length in Twins is Related to Birth-Weight Discordance Rachael Proctor MD Douglas Daly MD

    2. Background Discordant twin growth is believed by many to be a direct result of the process of twinning and of the inability of the uterine environment to provide for the increased demands of multiple fetuses.1 Discordant fetal growth has been reported to complicate 15-29% of twin gestations, with an associated increase in perinatal morbidity and mortality, especially for the smaller twin.2,3

    3. Review of the Literature There is no standardized definition of growth discordance, but most published studies have used a range of values from 15-25%.4-7 It has been reported that differences > 15% are more likely to be associated with growth retardation of one infant,7 whereas others have found the most clinically significant risk for adverse perinatal outcome to be present at 30% or more.6 Overall, the average birth-weight discordance among twins is in the range of 10%.4

    4. Review of the Literature Many studies have used ultrasound to predict and follow twin growth discordance using parameters such as BPD, AC, and EFW. Most have used late 2nd and 3rd trimester ultrasound to predict birth-weight discordance.8-12

    5. Review of the Literature One study reported that midtrimester (20-24 week) ultrasound has the greatest ability to predict gestations that will truly manifest a growth abnormality. If the midtrimester scan demonstrates normal growth, the chances are significantly decreased that abnormal growth on a subsequent scan is a true-positive finding.11

    6. Review of the Literature No studies of twin discordance that looked at first trimester ultrasound data could be identified.

    7. The Research Question: Do differences in crown-rump length identified at first trimester ultrasound result in an increased proportion of twins with birth-weight discordance?

    8. Study Design and Methods Retrospective cohort design All twins conceived between 1995 - 2001 who had delivered by November 2001 (78 twin pairs). All twins conceived through a single reproductive endocrinologist’s practice by assisted reproductive techniques or ovulation induction.

    9. Study Design and Methods Data collected prospectively included day of ovulation, early documentation of fetal cardiac motion, and ultrasonic measurement of crown-rump length on post-ovulatory day 41-58 (~8-10 weeks gestation). After ultrasound, patients were transferred to the care of their obstetrician, however, in most cases patients self-reported birth weights and length of gestation and the information was recorded in the patient chart. Otherwise, patients were contacted by phone or mail to obtain delivery information.

    10. Study Design and Methods We defined early discordance as a difference in crown-rump length of 2 or more millimeters at 8-10 week ultrasound, and refer to these pregnancies as “delayed”. Birth weights were recorded in grams, and the percentile discordance was calculated as a function of the larger twin.

    11. Statistical Analysis The data was analyzed for statistical significance of mean birth-weight discordance by t-test, and chi-square analysis was performed at levels of 15% and 20% discordance. We calculated the relative risk of birth-weight discordance in the presence of 1st trimester discordance with 95% confidence intervals.

    12. Results:

    13. Cohort size 78 viable twin gestations were discharged to the care of their obstetrician over the 6+ year study period. 3 later reduced to singleton pregnancies and were excluded from the study. There were no stillbirths. Three neonatal deaths (due to complications of prematurity). 74 sets dizygotic, 1 set monozygotic. No patients were lost to follow-up, for a total cohort size of 75 twin pairs.

    14. Statistically significant increase in birth-weight discordance > 15% when 1st trimester discordance (“delay”) is present

    15. Statistically significant increase in birth-weight discordance > 20% when 1st trimester discordance (“delay”) is present

    16. Small for gestational age infants 8 infants SGA (<10th %) All 8 in discordant pairs 7/8 discordant > 20% 5 in group with early concordance vs. 3 in group with early discordance Numbers are too small to draw any statistically significant conclusions between groups, however, this supports earlier studies demonstrating an increased risk of IUGR with discordance.7

    17. Analysis of Variance: Mean birth-weight discordance by group

    19. Relative Risk of birth-weight discordance (15% or more) when 1st trimester discordance (2 mm or more) is present, with 95% confidence intervals: 3.75 (2.09 - 6.74)

    20. Conclusions In twins, a difference in crown-rump length of 2 millimeters or more at first trimester ultrasound results in a statistically significant increase in the incidence of birth-weight discordance at levels of 15% and 20% or more, as well as a statistically significant increase in the mean birth-weight discordance.

    21. Conclusions We postulate that embryonic delay of one twin relative to the other results in a disproportionate allocation of uterine resources which predisposes the pair to birth-weight discordance. Our data suggests that in many cases discordant growth in twins is a much earlier intrauterine event than has previously been suggested.

    23. Discussion Strengths and weakness of study design: Prospective data collection with retrospective analysis eliminates bias in data collection. However, we were limited in the amount and type of data available for study. For example, would there be more discordant pairs if gestational sac size had been measured in addition to crown rump length?

    24. Discussion An unanswered question is whether there is any difference in the morbidity and mortality of discordant twins when this phenomenon of early discordance is present. More detailed data collection of obstetrical course and neonatal outcome, as well as a much larger cohort (perhaps multi-center) would be required to answer this question. We surmise that there would be no increase in morbidity and mortality above discordant twins without this early phenomenon.

    25. Discussion An additional unanswered question is whether this data translates to twins in the general population or is this phenomenon of early discordance unique to pregnancies with induced ovulation? One would expect a much higher mean birth-weight discordance in our cohort if this were the case. Overall, the mean discordance of 12.3% does not differ greatly from the mean discordance of ~10% reported in the literature for the general population.

    26. Discussion Our data supports the recommendation that in twin gestations, when early differences in crown-rump length are identified, these patients should be identified as at increased risk for birth-weight discordance, and more frequent ultrasound and clinical surveillance for growth discordancy may be warranted.

    27. Bibliography 1. Blickstein, I., and Lancet, M.: The growth discordant twin. Obstetrical and Gynecological Survey 43:509-515, 1988 2. Rodis, J., Vintzileos, A., et.al.: Intrauterine fetal growth in discordant twin gestations. J Ultrasound Med 9:443-448, 1990 3. Spellacy, W., et.al.: A case-control study of 1253 twin pregnancies from a 1982-1987 perinatal data base. Obstetrics and Gynecology 75: 168-171, 1990 4. Patterson, R., et.al.: What is twin birthweight discordance? Am J of Perinatology 7: 217-219, 1990 5. Blickstein, I., et.al.: Characterization of the growth-discordant twin. Obstetrics and Gynecology 70:11-15, 1987 6. Cheung, V. et.al: Preterm discordant twins: What birth weight difference is significant? Am J Obstetrics and Gynecology 172: 955-959, 1995 7. O’Brien, W., et.al.: Birth weight in twins: an analysis of discordancy and growth retardation. Obstetrics and Gynecology 67: 483-486, 1986 8. Caravello, J., et.al.: Sonographic Examination does not predict twin growth discordance accurately. Obstetrics and Gynecology 89: 529-533, 1997 9. Storlazzi, E., et.al.: Ultrasonic diagnosis of discordant fetal growth in twin gestations. Obstetrics and Gynecology 69: 363-367, 1987 10. Erkkola, R., et.al: Growth discordancy in twin pregnancies: a risk factor not detected by measurements of BPD. Obstetrics and Gynecology 66: 203-206, 1985 11. Grobman, W., et.al.: Positive predictive value of suspected growth aberration in twin gestations. Am J Obstetrics and Gynecology 181: 1139-1141, 1999 12. Blickstein, I., et.al.: Ultrasonic prediction of growth discordancy by intertwin difference in abdominal circumference. Int J Gynecology and Obstetrics 29: 121-124, 1989

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