Birth weight differences of more than 20% in twins is associated with poor outcomes for the mother and baby. Clinicians measure the estimated fetal weight differences by ultrasound before birth and compare it to differences in birth weight after the babies are born. In this review, we summarized data on whether the ultrasound measurements are accurate enough to predict birth weight differences in twins.
We searched medical databases to March 2019 for studies comparing ultrasound measurements to birth weight differences and we identified 39 studies. Twenty-two studies provided data on birth weight differences of 20% and 18 studies provided data on birth weight differences of 25%.
Quality of the evidence
We assessed the quality of individual studies using a tool called "Quality Assessment of Diagnostic Accuracy Studies" (QUADAS-2) and the overall quality by a recommended method called GRADE to find out the reliability of the evidence.
We found that ultrasound estimation of fetal weight differences compared to birth weight differences was not reliable. On average, ultrasound detected birth weight differences of 20% and 25% only half the time. The quality of evidence was very low.
There is insufficient evidence to support the use of ultrasound as the sole measure for detecting birth weight differences in twins, or poor outcomes. The diagnostic accuracy of other measures including amniotic fluid volume (the fluid surrounding the babies in the womb) or Doppler studies (which use sound waves to detect the movement of blood in the babies' blood vessels and the umbilical cord) in combination with ultrasound to inform clinical decisions needs to be evaluated. Future well-designed studies could also research the impact of whether the babies share a placenta (or not), the sex of the babies, and gestational age (time from woman's last menstrual period), in the diagnostic accuracy of ultrasound for estimated birth weight differences.
Very low-certainty evidence suggests that EFWD identified by ultrasound has low sensitivity but good specificity in detecting BWD in twin pregnancies. There is uncertain diagnostic value of EFWD; this review suggests there is insufficient evidence to support this index as the sole measure for clinical decision making to evaluate the prognosis of twins with growth discordance. The diagnostic accuracy of other measures including amniotic fluid index and umbilical artery Doppler resistive indices in combination with ultrasound for clinical intervention requires evaluation. Future well-designed studies could also evaluate the impact of chorionicity, sex and gestational age in the diagnostic accuracy of ultrasound for EFWD.
There is a need to standardize monitoring in obstetric research of twin pregnancies. Identification of birth weight discordance (BWD), defined as a difference in the birth weights of twins, is a well-documented phenomenon in twin pregnancies. Ultrasound for the diagnosis of BWD informs complex decision making including whether to intervene medically (via laser photo coagulation) or deliver the twins to avoid fetal morbidities or even death. The question is, how accurate is this measurement?
To determine the diagnostic accuracy (sensitivity and specificity) of ultrasound estimated fetal weight discordance (EFWD) of 20% and 25% using different estimated biometric ultrasound measurements compared with the actual BWD as the reference standard in twin pregnancies.
The search for this review was performed on 15 March 2019. We searched CENTRAL, MEDLINE (Ovid), Embase (Ovid), seven other databases, conference proceedings, reference lists and contacted experts. There were no language or date restrictions applied to the electronic searches, and no methodological filters to maximize sensitivity.
We selected cohort-type studies with delayed verification that evaluated the accuracy of biometric measurements at ultrasound scanning of twin pregnancies that had been proposed for the diagnosis of estimated BWD, compared to BWD measurements after birth as a reference standard. In addition, we only selected studies that considered twin pregnancies and applied a reference standard for EFWD for the target condition of BWD.
We screened all titles generated by electronic database searches. Two review authors independently assessed the abstracts of all potentially relevant studies. We assessed the identified full papers for eligibility, and extracted data to create 2 × 2 tables. Two review authors independently performed quality assessment using the QUADAS-2 tool. We excluded studies that did not report data in sufficient detail to construct 2 × 2 tables, and where this information was not available from the primary investigators. We assessed the certainty of the evidence using the GRADE approach.
We included 39 eligible studies with a median study sample size of 140. In terms of risk of bias, there were many unclear statements regarding patient selection, index test and use of proper reference standard. Twenty-one studies (53%) were of methodological concern due to flow and timing. In terms of applicability, most studies were of low concern.
Ultrasound for diagnosis of BWD in twin pregnancies at 20% cut-off
Twenty-two studies provided data for a BWD of 20% and the summary estimate of sensitivity was 0.51 (95% CI 0.42 to 0.60), and the summary estimate of specificity was 0.91 (95% CI 0.89 to 0.93) (8005 twin pregnancies; very low-certainty evidence).
Ultrasound for diagnosis of BWD in twin pregnancies at 25% cut-off
Eighteen studies provided data using a BWD discordance of 25%. The summary estimate of sensitivity was 0.46 (95% CI 0.26 to 0.66), and the summary estimate of specificity was 0.93 (95% CI 0.89 to 0.96) (6471 twin pregnancies; very low-certainty evidence).
Subgroup analyses were possible for both BWD of 20% and 25%. The diagnostic accuracy did not differ substantially between estimation by abdominal circumference and femur length but femur length had a trend towards higher sensitivity and specificity. Subgroup analyses were not possible by sex of twins, chorionicity or gestational age due to insufficient data.