The Institute for Advanced Medical Education is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians.

The Institute for Advanced Medical Education designates this educational activity for a maximum of 1 AMA PRA Category 1 Credit(s) TM. Physicians should only claim credit commensurate with the extent of their participation in the activity.

These credits are accepted by the American Registry for Diagnostic Medical Sonography (ARDMS).

Faculty:
Lyndon M. Hill, MD
Professor Obstetrics and Gynecology
Medical Director Ultrasound
Magee Women's Hospital
Pittsburgh, PA

Course: Twin Growth

Target Audience: Physicians, sonographers and others who perform and/or interpret obstetrical ultrasound.

System requirements: In order to complete this program you must have a computer with a recent version of Internet Explorer or Netscape, and a printer, which is configured to print from the browser.

For any questions or problems concerning this program or for problems related to the printing of the certificate please contact IAME at (914) 921-5700 or email us.

Estimated Time for Completion of tutorial: One hour
Date of Release and Review: April 24, 2006
Expiration Date: April 24, 2009

Disclosure: In compliance with the Essentials and Standards of the ACCME, the author of this CME tutorial is required to disclose any significant financial or other relationships they may have with the manufacturer(s) of any commercial product(s) or provider(s) of any commercial service(s) discussed in this program.

Dr. Lyndon Hill discloses no relevant financial relationships with commercial interests.

IAME discloses no relevant financial relationships with commercial interests. 

IAME Statement on Privacy and Confidentiality

Twin Growth
Lyndon M. Hill, MD
Professor Obstetrics and Gynecology
Medical Director Ultrasound
Magee Women's Hospital
Pittsburgh, PA

(DIRECT TO QUIZ)

Objectives
After completing this course, the participant should be able:

• Describe the etiology of and how to determine chorionicity and amnionicity of twin pregnancy.
• Recognize patterns of growth discordancy in monochorionic and dichorionic twins.
• Improve the outcome of twin pregnancy through the use of ultrasound to recognize the 'at risk' population.

Introduction
Twin gestations normally occur once in every 80 to 90 live births. Approximately 30% of spontaneous twins are monozygotic. The rate of monozygotic splitting is higher after artificial reproductive techniques 1 . The timing of division in monozygotic twins determines chorionicity and amnionicity (Table I) 2 . As noted in Table I, approximately 30% of monozygotic twins are dichorionic/diamniotic. While a first trimester ultrasound can reliably determine chorionicity and amnionicity, it cannot assess zygosity. A transvaginal ultrasound examination can visualize a corpus luteum in over 95% of early 1st trimester pregnancies. Tong et al 3 have proposed determining zygosity by evaluating the ovaries for corpus lutea - one corpus luteum in monozygotic twins; and two corpus lutea in dizygotic twins. Additional studies are required to test the accuracy of this technique.

+ Modified from reference 44

70% of spontaneous twins are dizygotic. Twin rates in the United States have increased from 18.8/1000 live births in 1975 to 26/1000 live births in 1995 4 . Over 50% of twins are currently the result of infertility treatment 5 . The rate of dizygotic twins is increased by certain hereditary and environmental factors: age >= 35 years; family history of dizygotic twins; parity; and race 6 .

Determining Chorionicity and Amionicity
Chorionicity can be predicted by determining the number of gestational sacs on a 5 to 6 week transvaginal ultrasound examination. The thickness of the tissue between the sacs is determined by the distance between implantation sites. A dichorionic twin pregnancy has a thick intervening membrane that is initially composed of amnion, chorion and decidua (Fig. 1). A monochorionic twin has a single surrounding chorion and only the amniotic membranes dividing the twins (Fig. 2). Hence, the number of embryos within a monochorionic cavity cannot be determined until embryonic poles with cardiac activity are visualized. Doubilet et al 7 have reported that an initial ultrasound between 5.0 and 5.9 weeks' gestation undercounts 24/213 (11%) of dichorionic twins and 6/7 (86%) monochorionic twins.

Figure 1 - 6.3 week diamniotic/dichorionic twin pregnancy.		Figure 2 - 9 week monochorionic/diamniotic (arrows) twins.
To view an enlargement, click on the image.

The success rate at distinguishing between monoamniotic and diamniotic monochorionic twins is, in part, menstrual age dependent 8 . During the first 11 weeks of pregnancy, the crown-rump length and cord length are approximately equivalent. The primary factors involved in determining cord length are sufficient space in the amniotic cavity and embryo/fetal movement resulting in tensile force being applied to the umbilical cord 9 . This relationship mandates that early in the first trimester the amniotic cavity and crown-rump length are also almost equal (Fig. 3) 10 . Hence, if two embryos are widely dispersed within a single chorionic cavity (Fig. 4), they must be diamniotic. When the embryos are in close proximity the determination of amnionicity must wait until 6-7 weeks'gestation, at which time the amnion can be clearly visualized on a transvaginal ultrasound examination (Fig. 5). If two closely approximated monochorionic twins at 6-7 weeks' gestation have a "v-shape", conjoined twins should be considered 11 .

Figure 3		Figure 4		Figure 5
6 week intrauterine pregnancy (arrow = amnion; y = yolk sac).		6.4 week monochorionic/diamaniotic twins a. crown-rump length of twin A (+. . .+); yolk sac (y) of twin B b. crown-rump length (x. . .x) of twin B		Monoamniotic 10 week twins (arrow = amnion).
To view an enlargement, click on the image.

As a general rule the number of yolk sacs is equal to the number amnions. However, Shen et al 12 reported that in 3/20 (15%) of monochorionic/diamniotic twins, only a single yolk sac was detected. Hence, monoamnionicity can be ruled out if there are 2 yolk sacs; a single yolk sac does not necessarily indicate a monoamniotic twin pregnancy.

Between 10 and 14 weeks' gestation the extension of placenta into the base of the inter-twin membrane ("twin peak sign")(Fig. 6), indicates a dichorionic twin pregnancy. The absence of a twin peak sign suggests that the twin pregnancy is monochorionic. As gestation advances, the twin peak sign becomes harder to identify. At 16-20 weeks' gestation the twin peak sign is still diagnostic for dichorionicity. However, its absence is not indicative of a monochorionic twin pregnancy and, therefore, does not exclude dizygosity 13 .

Figure 6 - Twin peak (between the arrows) indicating a diamniotic/dichorionic twin pregnancy. To view an enlargement, click on the image.

When two distinct placentas are visualized, the twins are usually dichorionic. However, 3% of monochorionic placentas are bilobed and may, therefore, appear as two distinct placental lobes 14 . Approximately half of dichorionic placentas are fused. Hence, the number of placentas cannot determine chorionicity in all cases 15 . Chorionicity rather than zygosity determines outcome. When monochorionic and dichorionic twins are compared, the former has a higher fetal loss rate prior to 24 weeks' gestation (12.2% vs. 1.8%); a higher perinatal mortality (28% vs. 1.6%); more deliveries prior to 32 weeks' gestation (9.2% vs. 5.5%); and more birth weights < 5th centile for both twins (7.5% vs. 1.7%) 16 .

Spontaneous fetal reduction in the 1st trimester
When twins are detected prior to 7 weeks' menstrual age, only 71% of pregnancies end in live twin neonates. The twin survival rate increases to 84% if the diagnosis is made between 7 and 9 weeks' menstrual age (Fig. 7) 17 .

Figure 7 - Diamniotic/dichorionic twin pregnancy at 7.8 weeks' gestation. Gestational sac B is significantly smaller than gestational sac A. The smaller twin subsequently died and the patient delivered a normal singleton. To view an enlargement, click on the image.

Term gestation in twins
61% of women carrying twins deliver by 38 weeks' gestation and 60% of women with singletons deliver by 40 weeks' gestation. After 37 weeks' gestation, twins have a higher stillbirth rate than singletons 18 . 6% of deliveries occur after 39 weeks and 42 weeks in twins and singletons, respectively. 37 to 38 weeks, therefore, appears to be the optimum gestational age for twins to deliver. Since mortality increases several-fold in twins after 39 weeks, elective delivery might be considered. A randomized controlled trial has not yet been undertaken to prove or disprove the efficacy of such a policy 19 .

In monochorionic twins the chromosomal make-up of the twins is the same. Placentation is also similar; the vascular communications between fetuses does not characteristically produce an imbalance in the first trimester. As a result, a significant disparity in crown-rump lengths between monochorionic twins has not been reported 2 .

If there is a discrepancy in crown-rump lengths between twins, the larger should be used for gestational age assessment 2 .

Growth of the head circumference in twins is not significantly different from singletons. Reece and co-workers 23 found that the biparietal diameter was significantly smaller in twins; they attributed this to head compression. In a more recent study Ong et al 24 reported that twin BPD's were larger than singletons between 24 and 31 weeks' gestation. After 32 weeks, the BPD showed a definite decline relative to singleton biparietal diameters.

Growth of the fetal long bones in twins is significantly different from singletons. However, this difference is not clinically relevant 25 .

The growth rate of the abdominal circumference in twins decreases significantly after 32 24 to 34 26 weeks' gestation.

Figure 8 - Twin discordancy. There is a 2.6 week difference in the size of the abdominal circumferences at 27 weeks' gestation. To view an enlargement, click on the image.

In monochorionic twins, discordant growth is usually due to placental vascular anastomoses that result in an imbalance of placental flow between the fetuses. Discordant growth in monochorionic twins is also associated with a velamentous cord insertion (Fig. 9) 20 . With dichorionic twins sub-optimal placentation of one twin may explain marked twin size discrepancies. A velamentous cord insertion in dichorionic twins is not associated with an increased risk of growth discordancy 20 . The growth of dichorionic twins that are destined to be discordant usually slows after 25 weeks' gestation. In monochorionic twins discordancy may be present by 20 weeks 32 .

Figure 9 - Velamentous cord insertion at 22 weeks' gestation. To view an enlargement, click on the image.

Intrauterine growth restriction in preterm discordant twins has been associated with a 7.7-fold increase in major neonatal morbidity 33 . Twins with normal growth remain undelivered significantly longer than twins in which one or both is growing slowly. Preterm delivery in twins is preceded by slowed or compromised growth of one or both twins 34 .

The 20 to 24 week scan has the best positive predictive value for predicting neonatal growth abnormality. If the 20 to 24 week scan has normal growth, the chance of abnormal growth later in gestation is significantly reduced, particularly in dichorionic twins 35 .

Naeye et al 37 recorded birth weights and developed percentiles for twins between 24 and 42 weeks' gestation based on a sample of 2,449 infants. Other groups 38 have developed similar twin percentile data. Some twin growth studies included twins with anomalies or those who died in the perinatal period; they were not, therefore, representative of "normal" growth 39 . Hence, the data utilizing twins is limited and may be derived from biased populations. Min 40 used both prenatal and postnatal weight measures to assess twin growth. Twins fell significantly below the 10th percentile for singletons between 28 weeks (based on predicted singletons in utero weight) and 34 weeks (based on singleton birth weight at term). Poor outcome for twins is best predicted by singleton rather than twin nomograms 41 .

The role of ultrasound is to recognize the "at risk" population of small fetuses. The two sonographic criteria that can be utilized are: 1) an estimated fetal weight < 10 th percentile using a singleton nomogram up to 32 weeks' gestation; and 2) an abdominal circumference below the 5 th percentile for singletons prior to 34 weeks' gestation. Serial assessment of growth is critical in establishing the diagnosis of intrauterine growth restriction in all but the most severe cases. If growth parallels the normal curve, the fetus is normal but small. A reduced growth velocity or a widening variance with expected growth increases the positive predictive value of a diagnosis of intrauterine growth restriction.

The etiology of perinatal loss with growth restriction is different between monochorionic and dichorionic twins. 33.3% of deaths in monochorionic twins is secondary to complications of twin-to-twin transfusion syndrome. The odds ratio of perinatal loss with growth restricted dichorionic twins is similar to singletons (0.91). Monochorionic growth restricted twins have a perinatal loss rate more than singletons (odds ratio: 2.45) 41 .

Figure 10 - A diagramatic sketch of a monochorionic twin pregnancy. Each gestational sac is divided into quadrants in order to obtain an amniotic fluid index for each twin.		Figure 11 - 17 week twin pregnancy. The amniotic fluid in the upper quadrants of one gestational sac is measured.
To view an enlargement, click on the image.

Conclusions
The individual assessment of growth in twins should be carried out with the same sonographic methods as singletons. Multiple abnormal parameters in fetal biometry, Doppler, or amniotic fluid volume indicate a more serious problem than a single abnormal parameter. In monochorionic twins growth discordancy of >= 15% has an equivalent perinatal risk to >= 30% weight discordancy in dichorinioc twins.

If there is an unbalanced vascular connection in monochorionic twins, both fetuses are at increased risk. With uteroplacental insufficiency in dichorionic twins, the smaller twin is at increased risk. All of the potential growth abnormalities outlined above highlight the importance of accurately determining chorionicity during the 1st ultrasound examination of a twin gestation.

References

1. Alikani M, Noyes N, Cohen J, Rosenwaks Z. Monozygotic twinning in the human associated with zona pellucide architecture. Hum Reprod 1994;9:1318-1321.
2. Sperling L, Tabor A. Twin pregnancy: the role of ultrasound in management. Acta Obstet Gynecol Scan 2001;80:287-299.
3. Tong S, Vollenhoven B, Meagher S. Determining zygosity in early pregnancy by ultrasound. Ultrasound Obstet Gynecol 2004;23:36-37.
4. Oleszczuk JJ, Keith DM, Keith LG. Projections of population-based twinning rates through the year 2100. J Reprod Med 1999;44:913-921.
5. Loos R, Derom C, Vlietinick R, Derom R. The East Flanderes Prospective Twin Survey ( Belgium ): A population-based register. Twin Research 1998;1:167-178.
6. Oleszczuk JJ, Oleszczuk AK , Keith DM, Keith LG. Twin and triplet births: facts, figures, and costs. Female Patient 2003;28:11-16.
7. Doubilet PM, Benson CR. "Appearing twin" undercounting of multiple gestations on early first trimester sonograms. J Ultrasound Med 1998;17:199-203.
8. Hill LM, Chenevey P, Hecker J, Martin JG. Sonographic determination of first trimester twin chorionicity and amnionicity. J Clin Ultrasound 1996;24:304-306.
9. Hill LM, DiNofrio DM, Guzick D. Sonographic determination of first trimester umbilical cord length. J Clin Ultrasound 1994;22:435-438.
10. Horrow MM. Enlarged amniotic cavity: a new sonographic sign of early embryonic death. AJR 1992;158:359-362.
11. Hill LM. The sonographic detection of early first trimester conjoined twins. Prenat Diagn 1997;17:961-963.
12. Shen O, Samueloff A, Beller U, Rabinowitz R. Number of yolk sacs does not predict amnionicity in early first-trimester monochorionic multiple gestations. Ultrasound Obstet Gynecol 2006;27:53-55.
13. Sepulveda W, Sebire NJ , Hughes K, Kalogeropoulos A, Nicolaides KH. Evolution of the Lambda or twin-chorionic peak sign in dichorionic twin pregnancies. Obstet Gynecol 1997;89:439-441.
14. Lopriore E, Sueters M, Middeldorp JM, Klumber F, Oepkes D et al. Twin pregnancies with two separate placental masses can still be monochorionic and have vascular anastomoses. Am J Obstet Gynecol 2006;194:804-808.
15. Filly RA, Goldstein RB, Callen PW. Monochorionic twinning: sonographic assessment. AJR 1990;154:459-469.
16. Sebire NJ , Snijders RJ, Hughes K, Sepulveda W, Nicolaides KH. The hidden mortality of monochorionic twin pregnancies. Br J Obstet Gynaecol 1997;104:1203-1207.
17. Sampson A, de Crespigny L Ch. Vanishing twins: the frequency of spontaneous fetal reduction of a twin pregnancy. Ultrasound Obstet Gynecol 1992;2:107-109.
18. Minakami H, Sato I. Re-establishing date of delivery in multifetal pregnancies. JAMA 1996;275:1432-1434.
19. Sairam S, Costelo K. Thilaganathan B. Prospective risk of stillbirth in multiple-gestation pregnancies: A population-based analysis. Obstet Gynecol 2002;100:638-641.
20. Kingdom JC, Nevi O, Murphy KE. Discordant growth in twins. Prenat Diagn 2005;25:759-765.
21. 21. Bartha JL, Ling Y, Kyle P, Southill PW. Clinical consequences of first-trimester growth discordance in twins. Eur J Obstet Gynecol Reprod Biol 2005;119:56-59.
22. Sebire NJ, D'Ercole C, Soares W, Nayai R, Nicolaides KH. Intertwin disparity in fetal size in monochorionic and dischorionic pregnancies. Obstet Gynecol 1998;91:82-85.
23. Reece EA, Yarkoni S, Abdalla M, Gabrielli S, Holford T et al. A prospective longitudinal study of growth in twin gestations compared with growth in singleton pregnancies. I. Fetal head. J Ultrasound Med 1991;10:439-444.
24. Ong S, Lim MN , Fitzmaurice A, Campbell D, Smith AP et al. The creation of twin centile curves for size. Br J Obstet Gynaecol 2002;109:753-758.
25. Reece EA, Yarkoni S, Abdalla M, Gabrielli S, Holford T et al. A prospective longitudinal study of growth in twin gestations compared with growth in singleton pregnancies. II. The fetal limbs. J Ultrasound Med 1991;10:445-450.
26. Rodis JF, Vintzileos AM, Campbell WA , Pinette MG, Nochimson DJ. Intrauterine fetal growth in concordant twin gestation. Am J Obstet Gynecol 1990;162:1025-1029.
27. Amaru RC, Bush MC, Berkowitz RL, Lapinski RH, Gaddipati S. Is discordant growth in twins an independent risk factor for adverse neonatal outcome? Obstet Gynecol 2004;103:71-76.
28. Blickstein I, Kalish RB. Birthweight discordance in multiple pregnancy. Twin Research 2003;6:526-531.
29. Ananth CV, Demissie K, Hanley ML. Birth weight discordancy and adverse perinatal outcomes among twin gestations in the United States : The effect of placental abruption. Am J Obstet Gynecol 1003;188:954-960.
30. Vergani P, Loentelli A, Ratti M, Scian R, Pozzi E et al. Preterm twins: what threshold of birth weight discordance heralds major adverse neonatal outcome? Am J Obstet Gynecol 2004;191:1441-1445.
31. Redman ME, Blackwell SC , Refuerzo JS, Krueger M, Naccasha N et al. The ninety-fifth percentile for growth discordance predicts complications of twin pregnancy. Am J Obstet Gynecol 2002;187:667-671.
32. Senoo M, Okamura K, Murotsuki J, Yaegashi N, Gehara S, Yajima A. A growth pattern of twins of different chorionicity evaluated by sonographic biometry. Obstet Gynecol 2000;95:656-661.
33. Yinon Y, Mazkereth R, Osentzweig N, Jarus-Hakak A, Schiff E et al. Growth restriction as a determinant of outcome in preterm discordant twins. Obstet Gynecol 2005;105:80-84.
34. Hediger ML, Luke B, Gonzalez-Quintero VH, Martin D, Nugent C et al. Fetal growth rates and the vary preterm delivery of twins. Am J Obstet Gynecol 2005;193:1498-1507.
35. Grobman WA , Parilla BV. Positive predictive value of suspected growth aberration in twin gestations. Am J Obstet Gynecol 1999;181:1139-1141.
36. Carlson NJ . Discordant twin pregnancy - a challenging condition. Cont OB Gyn 1989;34:100-116.
37. Naeye RL, Benirschke K, Hegstrom JWC, Marcus CC. Intrauterine growth of twins as estimated from liveborn birth-weight data. Pediatrics 1966;37:409-416.
38. Fliegner JR, Eggers TR. The relationship between gestational age and birth-weight in twin pregnancy. Aust NZJ Obstet Gynecol 1984;24:192-197.
39. Harrison SD , Cyr DR, Patten RM, Mack LA. Twin growth problems: causes and sonographic analysis. Sem Ultrasound, CT, and MRI 1993; 14:56 -67.
40. Min S-J, Luke B, Gillespie B, Min B, Newman RB et al. Birth weight reference for twins. Am J Obstet Gynecol 2000;182:1250-1257.
41. Hami Hon EF, Platt RW, Morin L, Usher R, Kramer M. How small is too small in a twin pregnancy? Am J Obstet Gynecol 1998;179:682-685.
42. Hill LM, Krohn M, Lazebnik N, Tush B, Boyles B, Ursiny JJ. The amniotic fluid index in normal twin pregnancies. Am J Obstet Gynecol 2000;182:950-954.
43. Moore TR, Cayle JE. The amniotic fluid index in normal human pregnancy. Am J Obstet Gynecol 1990;162:1168-1173.
44. Boklage CE. Twin Research 3: Twin Biology and Multiple Pregnancy 1981, pp 155-165. On the Timing of Monzygotic Twinning Events.

TAKE THE QUIZ

Our online courses are free of charge.

If you wish to earn CME credit then you will be asked to make payment by major credit card after successful completion of the quiz. The charge is $25 for one credit, $60 for four credits and $80 for eight credits. If you purchase multiple credits you may take them at any time.

If you are using a voucher please enter your email address exactly as you did when you made your original purchase.

IAME Statement on Privacy and Confidentiality