Fetal biometric parameters, twin type and birth weight difference

European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27–32

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Original Article

Fetal biometric parameters, twin type and birth weight difference A longitudinal study Christiane Charlemaine a , *, Michel Duyme a ,1 , Yves Ville b , Andre´ Aurengo c , Richard Tremblay d , Rene´ Frydman b , Jean-Claude Pons b,e a

´ ´ ´ ´ Unite´ de Recherches d’ Epidemiologie Genetique , INSERM U 155, Laboratoire d’ Anthropologie Biologique, Universite´ Paris VII, Tour 16, 3 e etage , Case 7041, 2 Place Jussieu, 75005 Paris 05, France b ´ ´ ˆ ´ ` , 92141 Clamart, France , Hopital Antoine Beclere Service de Gynecologie–Obstetrique c INSERM U 494, Paris, France d Department of Psychology, McGill University, Montreal, Canada e ´ ´ ´ ˆ Universitaire de Grenoble, France Service de Gynecologie–Obstetrique et Medecine de la Reproduction, Hopital Received 4 August 1999; accepted 21 December 1999

Abstract Objective: To examine the relationship between twin type, divided by zygosity, chorionicity, and birth weight difference [more or less than 15% intertwin weight difference at delivery] on fetal biometric measurements, including biparietal diameter, transverse abdominal diameter, head and abdominal circumferences (AC) and head and abdominal areas, femur length (FL), transverse cerebellar diameter, cerebellar circumference and cerebellar area at 18, 23, 28 and 32 weeks of gestational age. Study design: A prospective and longitudinal study was done on 75 twin pairs that did not present the twin–twin transfusion syndrome (150 fetuses) divided into birth weight difference group I (,15%) and group II ($15%). The twin pairs were divided into three groups: dizygotic, monochorionic, and dichorionic-monozygotic. In each group the differences in various ultrasound indices were evaluated in reference to discordant growth or concordant growth. Results: In our sample, there were no monozygotic twin pairs with dichorionic placentation with a birth weight difference of more than 15%. Significant associations of group and gestational age were found in dizygotic and monochorionic twins. Intrapair differences were significantly higher in group II ($15%) than in group I (,15%) for all fetal parameters studied except for AC, abdominal area and all cerebellar parameters. The value of these discrepancies increased according to gestational age except for FL. Significant interactions between group and zygosity type indicated that intrapair FL differences were more associated with group in monochorionic twins, whereas intrapair transverse cerebellar diameter differences were more associated with group in dizygotic twins. Conclusion: Our data show that most fetal biometric parameters are associated with birth weight discordancy. Abdominal area could be a relevant marker for twins with obstetric complications. Note that this is the first research that has studied a twin sample divided by both twin type and birth weight group.  2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Twins; Fetal growth; Biometric parameters; Birth weight discordance

1. Introduction The definition of normal growth and growth restriction *Corresponding author. Tel.: 133-1-4427-5052; fax: 133-1-44276972. 1 Present address: Univ. Paris V, CHI Poissy, Ecole de sages-femmes, 10 Rue du champ gaillard, 70303 Poissy, France.

as well as reference ranges for many ultrasound measurements are still poorly developed in twin pregnancies [1]. Standards of growth for twin gestation are available [2], however, growth analyses of twin gestations employing different methods focused mainly on differences between twins and singletons [3–5]. Some authors recommend the use of specific charts generated for twin pregnancies [2,6] whereas others conclude that growth charts for singletons

0301-2115 / 00 / $ – see front matter  2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0301-2115( 00 )00239-6

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C. Charlemaine et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27 – 32

may be applied accurately for twins [7–9]. The debate still exists regarding standards to be used to monitor intrauterine growth in twin pregnancies. Ultrasound has been used in twin pregnancies to determine the type of placentation, chorionicity, and amnionicity [6,10], and to screen for fetal anomalies, fetal demise, intrauterine growth retardation, and twin-to-twin transfusion. Intratwin weight discordancy in twin pregnancies remains a clinical as well as technical challenge. About 80% of all twin pairs differ by less than 15% in birth weight [10,11]. Consequently, some authors suggest that birth weight difference (BWD) exceeding 15% is a biologic and a logical limit [12], which reflects growth restriction in one twin even if birth weight is within normal limits for gestational age. For other authors, twin pairs are considered discordant if the difference in their birth weight exceeds 20% or more [13]. The aim of the study was to examine the relationship between twin type, — defined by zygosity (dizygotic and monozygotic), chorionicity (monochorionic and dichorionic), BWD (more or less than 15% weight difference at delivery) —, and fetal biometric measurements including biparietal diameter (BPD), transverse abdominal diameter, head circumference (HC) and abdominal circumference (AC) and their areas, femur length (FL), transverse cerebellar diameter, cerebellar circumference and its crosssectional area at 18, 23, 28, 32 weeks gestation.

zygotic and 30 monochorionic sets. In total, there were 78 females and 72 male fetuses. Fetal measurements were performed by different operators using a Toshiba Sono Layer 270 (France) with a 3.75-MHz sectorial probe. Fetal biometric parameters were measured on digital ultrasound images outside routine examination as previously described [14]. For the purposes of this investigation, we divided each twin type into BWD. To define BWD we used the percent definition of discordancy [15] calculated as the BWD expressed as percent of the larger twin’s birth weight. The cutoff value for discrepancy was set at 15%. Comparison of the growth pattern of biometric parameter measurements between group I (,15% BWD) and group II ($15% BWD) was performed at 18, 23, 28, and 32 weeks of gestation with analysis of intrapair difference means. First, mean and standard deviation (S.D.) of intrapair differences were calculated for each BWD group and by twin type (dizygotic, monochorionic and dichorionicmonozygotic) across gestational age. Then, two-way analyses of variance (ANOVAs) were used with BWD group, gestational age and zygosity type as independent variables and each of intrapair fetal parameter differences as dependent variables namely BPD, HC, head area, transverse abdominal diameter, AC, abdominal area, FL, and transverse cerebellar diameter, cerebellar circumference and its cross-sectional area.

2. Materials and methods

3. Results

From 1993 to 1998, fetal ultrasound examination data performed in twin pregnancies meeting the following inclusion criteria was studied: (1) lack of major obstetric complications or fetal anomalies; (2) at least three routine antenatal care examinations with ultrasound scan at this maternity; and (3) diamniotic twins both normal and alive at birth. Unlike-sex dizygotic twins were excluded with concern for gender homogeneity with monozygotic twins. Seventy-five twin pairs (150 fetuses) were therefore selected that had been included in a previous longitudinal study [13]. The diagnosis of zygosity was established using multiple red blood cell phenotypes (ABO, Rh, K, MN, Ss, Kidd, Duffy); the molecular biology techniques of amplifying DNA polymorphism at five loci were used in a second step. Monochorionic twins were classified as monozygotic. Among the dichorionic pairs, twins were considered to be dizygotic if they differed in one or more of the serological markers. They were considered monozygotic if they were alike for all the markers (serological and DNA) used. The placental chorionicity was determined: (1) by ultrasound examination before 14 weeks; (2) as well as by placental examination at delivery that was considered as the standard for diagnosis of chorionicity. Using these criteria, there were 35 like-sex dizygotic sets, ten dichorionic-mono-

Of the 75 twin pairs, 53 (71%) had a BWD of less than 15% while 22 (29%) had a BWD of more than 15%. Group I (,15% BWD) was composed of 20 dizygotic sets, ten dichorionic-monozygotic and 23 monochorionic sets, group II ($15% BWD) included 15 dizygotic and seven monochorionic sets of twins. In our sample there were no monozygotic twin pairs with dichorionic placentation with a BWD of more than 15%.

3.1. Intrapair differences during fetal period A total of 5088 fetal ultrasound measurements including: (1) BPD, transverse abdominal diameter, HC, AC and areas and FL obtained from 18 to 32 weeks gestation; and (2) transverse cerebellar diameter, cerebellar circumference and its cross-sectional area obtained from 18 to 28 weeks gestation, were performed on the 150 fetuses. First, we analyzed means and S.D.s of intrapair biometric differences calculated for each period of gestation (18, 23, 28, 32) by birth weight group for dizygotic, dichorionicand monochorionic-monozygotic twin pairs (Tables 1–4), and then for monochorionic and dizygotic twin pairs together (Table 5). Table 6 summarizes results of mean differences and interactions from ANOVAs for BWD group, gestational

C. Charlemaine et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27 – 32

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Table 1 Means and standard deviations of intrapair differences on BPD, HC, head area (HA), transverse abdominal diameter (TAD), AC, abdominal area (AA), and FL measurements by birth weight (,15% and $15%) differences for dizygotic twin pairs GAa (weeks)

18

BWD,15%, group I b Measurements (mm)

Mean S.D. Mean S.D. Mean S.D. Mean S.D.

23 28 32

BWD$15%, group II c Measurements (mm)

BPD

HC

HA

TAD

AC

AA

FL

BPD

HC

HA

TAD

AC

AA

FL

1.2 1.3 1.7 1.5 3.2 2.4 3.0 2.1

4.4 4.6 5.6 4.7 7.0 6.9 9.3 6.2

105.4 104.3 176.9 143.9 324.7 358.5 394.2 245.6

2.0 1.1 2.3 2.2 4.1 2.9 4.7 2.2

7.9 3.5 7.9 8.1 15.0 11.2 19.8 9.9

164.9 68.2 220.1 231.3 513.7 444.1 823.5 355.0

1.5 0.7 2.1 1.7 1.9 1.2 1.5 1.3

2.2 2.4 3.5 2.8 3.6 2.4 5.1 3.8

6.8 4.8 8.5 8.3 12.0 9.1 11.8 7.8

154.4 128.3 257.2 265.4 460.8 324.4 473.2 390.2

2.2 1.5 4.0 3.4 5.2 4.7 5.9 4.9

6.6 6.0 10.6 10.9 13.1 12.6 17.6 15.2

136.7 116.4 321.8 313.8 459.2 427.3 651.3 459.9

1.6 1.6 2.1 1.2 2.6 1.9 2.2 1.6

a

GA5gestational age. N51106 measurements in group I (40 at 18, 23 and 28 GA, and 38 at 32 GA for each parameter). c N5812 measurements in group II (30 at 18 and 23 GA, and 28 at 28 and 32 GA for each parameter). b

Table 2 Means and standard deviations of intrapair differences on BPD, HC, head area (HA), transverse cerebellar diameter (TCD), cerebellar circumference (CC), cerebellar area (CA), transverse abdominal diameter (TAD), AC, abdominal area (AA), and FL measurements by birth weight (,15%) differences for dichorionic-monozygotic twin pairs a GAb (weeks)

18

BWD,15%, group I c Measurements (mm)

Mean S.D. Mean S.D. Mean S.D. Mean S.D.

23 28 32

BPD

HC

HA

TCD

CC

CA

TAD

AC

AA

FL

1.5 1.1 2.2 1.3 2.1 1.9 2.5 1.6

3.6 3.7 2.5 1.1 6.6 4.6 9.8 8.7

79.1 75.0 96.2 27.2 244.4 184.2 391.0 303.0

0.5 0.5 1.3 1.2 2.1 2.0

3.2 2.2 4.8 2.1 6.2 6.9

15.2 10.3 44.9 49.3 85.4 61.9

1.6 1.0 1.8 1.4 4.8 2.5 4.3 3.0

4.9 3.1 4.9 4.7 14.0 10.3 16.1 12.6

94.2 67.1 137.0 131.3 488.0 377.7 669.7 510.5

1.4 1.0 1.3 1.2 1.6 0.9 2.3 1.4

a

There were no monozygotic twin pairs with dichorionic placentation over 15% BWD. GA5gestational age. c N5660 measurements in group I (20 [18 vs. 20 for HC, AH, TCD, CC, and CA] at 18 GA, 18 [14 vs. 18 for TCD, CC, and CA] at 23 GA, 18 [16 vs. 18 for BPD, HC, HA, TCD, CC, and CA] at 28 GA for each parameter, and 20 [18 vs. 20 for BPD, HC and HA] at 32 GA for TAD, AC, AA and FL). b

Table 3 Means and standard deviations of intrapair differences on BPD, HC, head area (HA), transverse abdominal diameter (TAD), AC, abdominal area (AA), and FL measurements by birth weight (,15% and $15%) differences for monochorionic twin pairs GAa (weeks)

18

BWD,15%, group I b Measurements (mm)

Mean S.D. Mean S.D. Mean S.D. Mean S.D.

23 28 32 a

BWD$15%, group II c Measurements (mm)

BPD

HC

HA

TAD

AC

AA

FL

BPD

HC

HA

TAD

AC

AA

FL

1.8 1.2 2.2 1.6 3.0 2.3 3.2 3.3

5.6 6.3 8.2 7.9 9.7 6.7 10.8 7.4

129.2 131.1 232.3 202.4 374.0 268.9 433.7 287.9

2.4 1.4 2.0 1.4 3.7 2.3 4.6 3.6

7.1 4.3 8.9 5.8 11.5 12.5 20.8 15.1

130.6 77.0 232.5 156.2 402.0 426.0 771.0 559.0

1.2 0.9 1.6 0.9 1.7 1.5 1.3 1.1

2.4 1.6 2.4 1.2 4.2 2.4 4.5 2.3

6.2 4.3 10.7 4.2 10.7 6.9 18.9 14.0

146.3 92.2 328.6 129.1 394.4 290.1 746.3 573.8

3.4 3.6 2.8 3.2 5.6 4.6 8.4 6.9

8.5 9.1 12.1 14.5 17.8 15.7 24.2 11.7

163.8 172.9 327.5 373.5 629.9 509.1 922.4 555.9

1.9 1.9 2.3 1.4 2.8 2.4 3.5 3.0

GA5gestational age. N51186 measurements in group I (40 at 18 and 32 GA, and 44 at 23 GA, and 46 at 28 GA [except HA and AA with 44] for each parameter). c N5378 measurements in group II (14 at 18, 23 and 28 GA and 12 at 32 GA for each parameter). b

C. Charlemaine et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27 – 32

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Table 4 Means and standard deviations of intrapair differences for transverse cerebellar diameter (TCD), cerebellar circumference (CC) and cerebellar area (CA) measurements by birth weight (,15% and $15%) differences for monochorionic (MC) and dizygotic (DZ) twin pairs GAa (weeks)

18

Mean S.D. Mean S.D. Mean S.D.

23 28

MC — BWD,15%, group I b Measurements (mm)

MC — BWD$15%, group II c Measurements (mm)

DZ — BWD,15%, group I d Measurements (mm)

DZ — BWD$15%, group II e Measurements (mm)

TCD

CC

CA

TCD

CC

CA

TCD

CC

CA

TCD

CC

CA

0.7 0.5 1.5 1.4 1.7 1.4

3.4 3.1 5.2 4.3 4.5 4.5

22.3 23.8 41.6 34.0 57.2 43.6

0.6 0.3 1.0 0.6 0.8 0.8

1.7 0.8 4.2 2.4 3.6 2.7

4.8 2.7 40.0 42.5 37.3 10.0

0.9 1.1 1.6 1.2 1.7 1.9

3.4 2.5 5.5 3.7 5.5 3.8

28.0 17.2 41.8 23.2 78.8 54.9

1.2 0.9 1.8 1.4 3.1 2.5

3.2 1.5 6.2 4.2 8.6 8.3

20.5 15.7 51.0 47.4 88.3 79.0

a

GA5gestational age. N5300 measurements c N5108 measurements d N5318 measurements e N5220 measurements b

in in in in

MC group I (36 at 18 GA, 34 at 23 GA, and 30 at 28 GA for each parameter). MC group II (12 at 18 GA, 14 at 23 GA, and 10 at 28 GA for each parameter). DZ group I (38 at 18 GA, 36 at 23 GA, and 32 at 28 GA for each parameter). DZ group II (26 [28 vs. 26 for TCD] at 18 and 23 GA, and 20 at 28 GA for each parameter).

Table 5 Means and standard deviations of intrapair differences on BPD, HC, head area (HA), transverse abdominal diameter (TAD), AC, abdominal area (AA), FL, transverse cerebellar diameter (TCD), cerebellar circumference (CC) and cerebellar area (CA) measurements by birth weight (,15% and $15%) differences for dizygotic and monochorionic twin pairs together GAa (weeks)

18

BWD,15%, group I Measurements (mm)

Mean S.D. Mean S.D. Mean S.D. Mean S.D.

23 28 32 a

BWD$15%, group II Measurements (mm)

BPD

HC

HA

TAD

AC

AA

FL

TCD

CC

CA

BPD

HC

HA

TAD

AC

AA

FL

TCD

CC

CA

1.5 1.2 2.0 1.6 3.1 2.3 3.1 2.7

5.0 5.5 6.9 6.6 8.4 6.9 10.1 6.8

117.0 118.0 206.0 177.0 351.0 312.0 414.0 265.0

2.2 1.3 2.1 1.8 3.9 2.6 4.6 3.0

7.5 3.9 8.4 6.9 13.1 11.9 20.4 12.7

148.0 74.0 227.0 193.0 454.0 433.0 796.0 468.0

1.3 0.8 1.8 1.4 1.8 1.4 1.4 1.2

0.8 0.8 1.6 1.3 1.7 1.6

3.4 2.7 5.4 3.9 5.0 4.2

25.2 20.6 42.7 28.5 68.3 50.1

2.3 2.1 3.2 2.5 3.8 2.3 5.0 3.3

6.6 4.6 9.2 7.2 11.6 8.2 13.9 10.2

152.0 116.0 280.0 230.0 439.0 308.0 555.0 455.0

2.6 2.4 3.6 3.3 5.3 4.5 6.7 5.5

7.2 6.9 11.1 11.9 14.7 13.5 19.6 14.3

145.0 133.0 324.0 325.0 516.0 451.0 733.0 492.0

1.7 1.6 2.1 1.2 2.7 2.0 2.6 2.1

1.0 0.8 1.5 1.2 2.4 2.3

2.7 1.5 5.5 3.7 6.9 7.3

15.5 14.9 47.3 45.1 71.3 68.3

GA5gestational age.

Table 6 Differences of means and interactions from ANOVAs for BWD group, gestational age, and zygosity type (dizygotic vs. monochorionic twins) as independent variables with each fetal ultrasound parameter, with a P-value ,0.05 a Difference of means Independent variables 1 Group 2 GA 3 Zygosity Interactions 1 vs. 2 1 vs. 3 2 vs. 3 13233 a

BPD

HC

HA

TAD

AC

AA

FL

TCD

CC

CA

Y Y N

Y Y Y

Y Y N

Y Y N

N Y N

N Y N

Y N N

N Y Y

N Y Y

N Y Y

N N N N

N N N N

N N N N

N N N N

N N N N

N N N N

N P,0.03* N N

N P,0.02** N N

N N N N

N N N N

GA5gestational age; N5No; Y5Yes for significant difference of means or interactions. * Interaction with BWD in monochorionic twin pairs. ** Interaction with BWD in dizygotic twin pairs.

C. Charlemaine et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27 – 32

age at ultrasound examination, and zygosity type (dizygotic vs. monochorionic twins) as independent variables with each of the fetal ultrasound parameters. Results of analyses indicated that intrapair differences were significantly higher in group II ($15% BWD) than in group I (,15% BWD) regardless of gestational age ultrasound examination and zygosity type for all fetal biometric parameters, except AC, abdominal area and all cerebellar parameters. Regarding the relationship with gestational age ultrasound examination, results showed a significant increase with gestational age ultrasound examination of intrapair differences for all fetal biometric parameters except for FL. Concerning the relationship with zygosity type, results showed that intrapair differences were higher in monochorionic twins than in dizygotic twins for HC, they were weaker in monochorionic twins for all cerebellar parameters than in dizygotic twins. Significant interactions between BWD group and zygosity type indicated that intrapair FL differences were more associated with BWD group in monochorionic twins, whereas intrapair transverse cerebellar diameter were more associated with BWD group in dizygotic twins. To sum up, according to BWD group, HC and all cerebellar parameters were not relevant. We found a link: (1) with HC in monochorionic twins but not in dizygotic twins; (2) with all cerebellar parameters in dizygotic twins but not in monochorionic twins. According to growth development, intrapair FL differences did not develop in both twin types.

4. Discussion Our results indicate significant associations of BWD group and gestational age in dizygotic twins and monochorionic twins. Intrapair differences were significantly higher in group II ($15% BWD) than in group I (,15% BWD) for all fetal parameters studied except for AC, abdominal area and all cerebellar parameters. The value of these discrepancies increased according to gestational age except for FL. BPD has been the most widely studied growth parameter in twins [4]. A difference of more than 4 mm [16] (.28 weeks gestation) or 5 mm or more in intrapair BPDs [17] (.30 weeks of gestation) is associated with discordance and poor fetal outcome. Shah et al. [9] consider an intrapair difference of 10% or more to be statistically significant. In our study on BPD, we found a significant statistical effect of group, which shows that BPD is to a large extent linked to fetal weight. Intrapair BPD difference in BWD group II ($15% BWD) was 5.1 mm for dizygotic twins and 4.5 mm for monochorionic twins (Tables 1 and 3). However, authors have shown that there is no linear correlation between BPD and birth weight. BWDs of greater than 25% could be found with BPD

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differences ranging from 0 to 16 mm, whereas weight differences of as little as 5% could be seen with a BPD difference measuring 11 mm [11]. Erkkola et al. [18] found a birth weight discordancy of 30% with a BPD difference of only 1 mm. Campbell and Wilkin [19] were one of the first teams to publish studies using fetal AC. On the commonly used single ultrasonic parameters, the intrapair difference in AC has been suggested to be more useful than BPD difference in predicting discordant fetal growth [9]. The high sensitivity of the intrapair difference in AC occurs as a result of the AC discrepancy being the first measurement to become abnormal in fetuses with asymmetric growth retardation, thus no important difference in AC or a difference of less than 20 mm seems predictive of concordant fetal growth [20]. We found in this study no significant link with fetal AC. Those results do not support the suggestion of others authors that intrapair AC difference is more useful than BPD difference in predicting discordant fetal growth in twins. On the other hand, this result could be one of the more interesting of this study. Indeed, the other studies included fetus with obstetric complications and anomalies. They were excluded from our sample. One can think that intrapair AC will be a marker specifically important in the case of fetal pathologies. By far the most commonly used measurement is the BPD, but fetal FL has also been included in the ultrasonic determination of intrauterine growth retardation. Storlazzi et al. [20] evaluated several ultrasonographic indices as predictors of discordant fetal growth and showed than a difference in FL of 5 mm or more had a positive predictive value of 75%. A slightly lower positive predictive value of 63% was calculated in the study by Divon and Weiner [10]. However, Abramowicz et al. [21] observed that FL has no abnormalities of growth even in very small or very large fetuses and there was no statistical difference, as compared with normally grown fetuses. In the present study, intrapair FL differences were linked to BWD group. However, these intrapair differences increased according to gestational age for BPD and transverse abdominal diameter but not for FL in both monochorionic and dizygotic twins. Although a number of studies have been performed on BPD, transverse abdominal diameter and FL in twin pregnancy, few authors reported on transverse cerebellar diameter in the twin population. It is noted that unlike other biometric variables transverse cerebellar diameter measurements are relatively not linked by discordancy in twin growth [22,23]. Hill et al. [24] observed that in the majority of cases, even with a greater than 20% difference in birth weight, the transverse cerebellar diameters are not significantly different. However, Lettieri et al. [25] reported that intrauterine growth retardation affects cerebellar growth in twin gestations. According to BWD group, our finding are consistent

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C. Charlemaine et al. / European Journal of Obstetrics & Gynecology and Reproductive Biology 93 (2000) 27 – 32

with authors having observed no significant intrapair differences in transverse cerebellar diameter. Notwithstanding, we found a significant relationship with intrapair transverse cerebellar diameter differences in dizygotic twin pairs in BWD group II ($15% BWD). This study found a close correlation between most of the fetal biometric parameters and birth weight group, whatever the twin type. Our data show that most biometric parameters measured sonographically were affected by fetal weight: the differences between birth weight groups were statistically significant. Intrapair abdominal circumference differences which are not linked to birth weight discordancy in our fetus sample without obstetric complications could be a marker specifically important in the case of fetal pathologies. Note that this is the first research that has studied a twin sample divided both by twin type and birth weight group.

[8]

[9]

[10] [11]

[12]

[13]

[14]

5. Condensation Most of the fetal biometric parameters are associated with birth weight discordancy, whatever the twin type.

[15] [16]

Acknowledgements

[17]

This work was supported by a grant from the Institut ´ National de la Sante´ et de la Recherche Medicale, Paris, France.

[18]

[19]

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