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The use of three-dimensional ultrasound for fetal gender determination in the first trimester

¹ Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Royal Free Hospital, Pond Street, London NW3 2QG and ² Department of Primary Care & Population Sciences, Royal Free & University College Medical School, Rowland Hill Street, Hampstead, London NW3 2PF, UK

Correspondence: Mr D L Economides

	Abstract

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The objective of this study was to examine the accuracy of fetal gender prediction at a routine first trimester scan using three-dimensional (3D) ultrasound. 200 women were recruited for this study and they agreed to have a transvaginal scan for their routine first trimester scan for fetal anatomy and nuchal thickness measurement. 3D volumes were obtained and stored. Two examiners independently reviewed all the volumes and recorded their diagnosis of fetal gender and measured the angle between the genital tubercle and the skin overlying the sacrum. After studying the 3D volumes both examiners recorded a diagnosis of male or female in 150 cases (81.5%). In 34 cases (18.5%) either both (n=21) or one of them (n=13) could not comment on fetal gender by studying the saved volume. From these 150 cases correct prediction of fetal gender by both examiners was achieved in 85.3% of cases. In 6.7% of cases both examiners predicted the wrong gender while for the rest 8% of cases each examiner assigned different gender to the fetus (k=0.84; standard error 0.045). Angle measurements performed from the saved 3D volumes were highly reproducible. Gestational age did not affect the accuracy of gender identification. This study demonstrates that 3D ultrasound can be an effective and fast way of identifying fetal gender in the first trimester. The advantages of 3D ultrasound stem from its ability to virtually reproduce all required views.

	Introduction

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First trimester scanning has evolved from determining fetal viability to a comprehensive anatomical [1] and chromosomal abnormality screening examination [2]. Accurate prediction of fetal gender at this early phase of pregnancy has always been a challenge and a number of studies have tried to assess the value of such an examination. The main perceived advantage of early pregnancy fetal gender determination is the reduction of invasive procedures for the diagnosis of X-linked genetic abnormalities. The studies performed to date agree that the best results in determining the fetal gender are achieved after 12 weeks gestation [3, 4] and two views have been proposed, namely sagittal [5–7] and axial [8].

In this study we used a scanner capable of three-dimensional (3D) volume acquisition. The ability to virtually manipulate the acquired volumes in order to achieve the desired views is the major advantage of 3D scanning. We tested the hypothesis that the use of 3D ultrasound would provide accurate fetal gender prediction during a routine first trimester scan without significantly increasing the actual scanning time.

	Methods

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200 women were recruited at their booking appointment to participate in this study. Inclusion criteria were a live singleton pregnancy with an estimated gestational age 11–14 weeks.

The study was conducted at the fetal medicine unit by two clinical research fellows experienced in first trimester scanning and the use of 3D ultrasound. Ethical approval was obtained from the local hospital committee.

The participants agreed to have a transvaginal scan for their routine first trimester scan for fetal anatomy and nuchal thickness (NT) measurement. Two volumes where obtained using the Kretz (Zipf, Austria) S-VDW 7 MHz 3D transvaginal probe. The probe uses an integrated motor to produce a slow tilting movement of a conventional 2D probe and automatically performs the volume scan. In that way a continuous series of 2D information is obtained and with the use of specialized software 3D reconstruction is possible (Figures 1 and 2). As the time and speed variables of the probe movement are known the software can provide not only surface reconstruction but also virtual scanning in all dimensions. The only requirements for the volume acquisition was for the fetus to be at rest and no extra time was allowed for the fetus to change to a more favourable position for scanning. However, the probe was moved in order to obtain the optimal view of the sacral region. The time needed to obtain the volumes was recorded. The volumes were stored in our unit PC and were examined at a later time. The results were not communicated to the parents as agreed at the time of consent.

View larger version (96K): [in this window] [in a new window]   Figure 1. (a) 3D reconstruction of external genitalia. (b) Sagittal and (c) axial views of the genital tubercle in a female fetus. Arrows indicate the genital tubercle.

View larger version (101K): [in this window] [in a new window]   Figure 2. (a) 3D reconstruction of external genitalia. (b) Sagittal and (c) axial views of the genital tubercle in a male fetus.

The outcome of the pregnancies and clinical confirmation of fetal gender were obtained from the computerized database of our hospital and patients records.

The statistical package SPSS (Chicago, IL) was used for all statistical calculations. In order to measure the degree of agreement between the two examiners we calculated Cohen's k.

	Results

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From the 200 women recruited in this study the pregnancy outcome was available in 186 cases (93%). We excluded from analysis two patients who had a miscarriage after their first trimester scan.

The mean age of the participants was 32 years (standard deviation (SD) 5.16 years). The mean crown rump length (CRL) was 66 mm (SD 8.4 mm, min: 48 mm, max: 84 mm) which corresponds to a gestational age of 13 weeks. The fetal gender was recorded at birth as male infant in 103 cases (55.9%) and female infant in 81 cases (44.1%). There were no obvious external genital abnormalities recorded at birth in any of the cases.

After studying the 3D volumes both examiners recorded a diagnosis of male or female in 150 cases (81.5%). In 34 cases (18.5%) either both (n=21) or one of the examiners (n=13) could not comment on fetal gender by studying the saved volume. For examiner A in 16.3% and for examiner B in 13.6% of cases fetal gender could not be assigned. From these 150 cases correct prediction of fetal gender by both examiners was achieved in 128 cases (85.3%). In 10 cases (6.7%) both examiners predicted the wrong gender while for the other 12 cases each examiner assigned different gender to the fetus (8%). Table 1 summarizes each examiner's performance.

Angle measurements performed from the saved 3D volumes were highly reproducible. The mean difference was 1.3°, SD 12.1, standard error of the mean 0.98 (95%CI -0.7–3.2). The mean angle for male fetuses was 55 degrees (SD 14; examiner A) and 56 degrees (SD 14.4; examiner B). The respective values for female fetuses were 17 degrees (SD 7.4; examiner A) and 21 degrees (SD 9.1; examiner B).

Gestational age did not affect whether the gender was identified correctly (Figure 3). For the cases that the examiner could not comment on fetal gender the reasons were:

1. Fetal position, most frequently the thighs, were close together or the fetus was in cephalic presentation with the fetal sacral area too far from the probe (examiner A: 15 cases, examiner B: 10 cases).
   
2. Umbilical cord between the legs (examiner A: 7 cases, examiner B: 7 cases).
   
3. Unsatisfactory volumes due to movement artefacts (8 cases for both examiners).
   

View larger version (23K): [in this window] [in a new window]   Figure 3. Examiners performance in predicting fetal gender at 11, 12 and 13 weeks gestation.

	Discussion

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The advantages of 3D ultrasound stem from its ability, using specialized software, to virtually reproduce all required views. With minimal manipulation the sagittal and axial sections of the fetus in optional plane were easily obtained. The volume acquisition was performed after obtaining a reasonable view of the sacral region independent of the fetal position. The transvaginal approach was selected due to better image resolution achieved in all planes by the transvaginal ultrasound probe. One of the recognized disadvantages of transvaginal imaging is the limited manoeuvrability [9] of the probe and this was overcome by the 3D reconstruction. We have already shown that examination of 3D stored volumes have the required accuracy to perform a first trimester anatomy examination [10] and as the information needed could be stored and subsequently examined, the actual scanning time could be reduced.

Lev-Toaff et al [11] claimed 100% accuracy in diagnosing fetal gender using a similar technique. However, in that study the number of fetuses examined in the first trimester was small. Using 2D ultrasound the gestational age at the time of scan appears to be one of the main predictors of the accuracy of fetal gender identification. Whitlow et al [4] reported improving accuracy as the pregnancy advanced from 11 weeks to 14 weeks. In the present study there was no significant difference attributable to gestational age in either the ability to predict fetal gender or the accuracy of the prediction. This is explained by the generally excellent 3D views of the fetus obtained at 11 weeks with the transvaginal probe. The two most frequent problems encountered at the time of the volume acquisition were the thighs close together or the genital tubercle being too far from the probe. These problems can be avoided if more time is allocated for the scan and the operator checks the position of the fetal lower limbs before the volume is saved. However, as we wanted to study the performance of 3D scanning in a routine clinical setting, scanning time was restricted. Another potential limitation in achieving 100% accuracy is the change of the angle of the penis with the body. Bronstein et al [5] suggested that the penile corpora carvenosa are permanently congested resulting in constant erection of the fetal penis. However, Perdeira et al [12] commented on the effect of fetal erection in predicting fetal gender and demonstrated a change of angle during imaging.

Although up to 75% of mothers prefer to know the fetal gender [13] the main clinical indication for early fetal gender identification is pre-natal diagnosis of X-linked disorders. For this an accuracy of 100% would be required. However, we believe that there is a place for first trimester fetal gender determination. It could be argued that provided the parents understand the limitations of fetal gender identification they could opt to defer invasive testing.

	Acknowledgments

 
We would like to thank Ms Samara Abrahams for her invaluable administrative support in performing this study.

Received for publication August 20, 2002. Revision received March 12, 2003. Accepted for publication April 9, 2003.

	References

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