Ultrasound diagnosis of birth weight imbalance in twin pregnancies

Birth weight imbalance (BWD), defined as the difference in twin birth weights, is a well-documented phenomenon in twin pregnancies. When twins are suitable for the gestational age, inconsistent growth can occur as physiological BWD, or physiological BWD may occur when at least one fetus is small at gestational age.

BWD is one of the major risk factors for poor fetal, neonatal and maternal outcomes. Demissie et al. Found that the ratio of fetal deaths ranged from 1.26 to 12.75 and the ratio of neonatal deaths ranged from 1.02 to 3.43, depending on the degree of birth weight difference. Kilic and colleagues report a higher prevalence of mortality, sepsis, polycythemia, hypoglycemia, anemia and respiratory distress syndrome among different twins. Wen et al. Report a higher likelihood of maternal hypertension, eclampsia, and other medical complications associated with bilateral BWD. BWD occurs in about 15% to 30% of twin pregnancies based on chorionicity, a degree of inconsistency, and the threshold for use. Fetal growth abnormalities are associated with a variety of diseases that affect the health of the fetus or newborns, such as twin transfusion (TTT), chromosomal aberrations or structural defects. Excluding these conditions, uncoordinated growth itself is considered an independent factor associated with poor perinatal outcomes. BWWs of 20% and 30% are the most common thresholds used in the literature to identify adverse perinatal outcomes. A recent study showed that a BWD greater than or equal to 30% was significantly associated with perinatal adverse events.

Several mechanisms have been proposed to uncoordinate the growth of the fetus exposed to the same intrauterine environment. For single-fetus (MC) twin pregnancies, the mechanism can be explained by conditions such as TTT and intrauterine growth retardation (IUGR). Both conditions lead to higher fetal and neonatal mortality risks compared to twins. Technology can be used to manage TTT, including amniotic fluid reduction and ostomy, so monitoring usually starts after 20 weeks.

In the case of DC twin pregnancies, IUGR and placental pathology can lead to inconsistent growth. Other suggested mechanisms of BWD are abnormal umbilical cord insertion and placental insufficiency. These conditions are not treatable; the primary management is therefore to determine the health of the fetus and to determine the optimal delivery time to save the unborn twins.

The incidence of disharmonious twins was higher than that of identical twins (odds ratio 5.69; 95% confidence interval (CI) 3.24 to 10.00). If this inconsistency increases to more than 30% of body weight differences and/or if TTT is suspected, the clinician must decide whether to deliver twins to avoid fetal illness or even death. The clinical relevance of other BWD thresholds (eg, 25%, 20%) and estimated fetal birth weight measured by ultrasound (eg, biparietal diameter) associated with poor outcome has not so far been systematically reviewed. Therefore, we aimed to determine the diagnostic accuracy of the available ultrasound measurements used to determine the estimated BWD for various thresholds of twin pregnancies.

Ultrasound has been an important tool in the diagnosis of BWD since 1972. Diagnostic ultrasound(usg ultrasound) is a complex electronic technique that utilizes high-frequency sound pulses to produce images. In the past, the only technique available was abdominal palpation, which was very poor for detecting developmental inconsistency. Radiological examination is not recommended because it is not safe for the fetus.

The diagnosis of BWD can be done in three modes of ultrasound:

A series of diagnostic two-dimensional (2D) ultrasound exams to assess twin fetal growth, identify chorion, and diagnose umbilical cord, amniotic layers, congenital abnormalities, and TTT problems;

Doppler ultrasonography to detect abnormal blood flow patterns in the fetal/placental circulation may indicate poor fetal prognosis;

Three-dimensional (3D) ultrasound is used to promote placental assessment, such as quantitative and qualitative assessment of surface imaging and volume measurement, placental vascularization, and blood flow.

The standard diagnostic test for inconsistent growth is ultrasound. Incomparable growth can be estimated by measuring the length of the buttocks (CRL: distance measured from overhead to buttocks) at or before 12 weeks of gestation. Thereafter, other ultrasound measurements, such as biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL), were used to estimate fetal weight and calculate BWD during the second trimester and the third trimester

It is not yet clear whether BWD can be accurately predicted in the first trimester of pregnancy, whether ultrasound scan machine parameters are more reliable during each pregnancy to assess inconsistency, and whether any particular ultrasound modality (eg 2D or 3D) is superior to other ultrasound modalities.

In addition to BWD, chorionic sex information is necessary for the management of twin pregnancies for the following reasons.

Due to co-vascularization, MC twin fetuses have a higher morbidity and mortality.

If there is a single evidence of chorionic irritation in early ultrasound screening, the screening should be continued during the second and third trimesters.

Early ultrasound testing reduced the differential diagnosis of underlying causes such as fetal sharing and subsequent vascular anastomosis, which is the underlying cause of fetal BWD. DC twin pregnancies are expected to have a better prognosis because they do not share the placenta.

Interim gestational ultrasound screening for 16 to 24 weeks provides other measures such as exchange, maximum and vertical amniotic pockets and identification of dividing membranes, umbilical arteries and potential Doppler studies.

After 24 weeks gestation, ultrasound in late pregnancy is intended to detect inconsistencies or insufficient fetal growth. If one of the twins is smaller than the gestational age and is not suitable for the gestational age, identification of inconsistent twins is crucial.

It should be noted that the use of these diagnostic tests from one pregnancy to another is overlapping.