Selective IUGR in Twins: Doppler Types and Ultrasound Monitoring

Clinical Essence

Selective intrauterine growth restriction in twins is not just a 'small fetus,' but a combination of the small size of one twin and intertwin discordance, which alters the risk of antenatal death, neurological damage, and iatrogenic prematurity. The updated ISUOG 2025 guidelines on ultrasound in twins emphasize early determination of chorionicity, regular biometry, and Doppler stratification, especially in monochorionic twins.

It is practically important to separate two tasks: diagnosing discordant growth and determining the hemodynamic type. In dichorionic twins, the approach is closer to managing singleton IUGR, considering the second twin and gestational age. In monochorionic twins, the prognosis is more dependent on placental anastomoses, so classification by the umbilical artery of the smaller twin has independent significance.

Calculation of Mass Discordance

Discordance in estimated fetal weight is calculated from the weight of the larger twin:

Discordance, % = (EFW of the larger twin − EFW of the smaller twin) / EFW of the larger twin × 100.

EFW is the estimated fetal weight based on standard biometry. The threshold of ≥25% in ISUOG guidelines is used as a clinically significant discordance requiring in-depth assessment and typically referral to a fetal medicine center. Isolated weight difference without percentile, abdominal circumference, and Doppler assessment is insufficient for comprehensive risk stratification.

Diagnostic Criteria Provided by ISUOG

ISUOG 2025 presents a consensus approach: selective IUGR can be diagnosed with extremely low estimated fetal weight of one twin or a combination of several criteria. In clinical protocol, this is conveniently used as a checklist to not miss the early form of the condition.

Types of Selective IUGR in Monochorionic Twins

The classification is primarily applied to monochorionic diamniotic twins and is based on the nature of end-diastolic blood flow in the umbilical artery of the smaller twin. It reflects not only placental resistance but also the influence of intertwin vascular anastomoses.

What to Include in the Ultrasound Protocol

The minimum protocol when suspecting selective IUGR should be structured. The report should indicate chorionicity and amnionicity, position of each twin, biometry with estimated weight, percentiles, abdominal circumference, amount of amniotic fluid for each twin, discordance in EFW in percentages, and Doppler indices.

For Doppler, the key vessels are the umbilical artery, middle cerebral artery, and ductus venosus. In monochorionic twins, the nature of blood flow in the umbilical artery of the smaller twin must be formulated as type I, II, or III if the criteria for selective IUGR are met.

Basic Monitoring Schedule for Twins

According to ISUOG, the frequency of planned ultrasound monitoring depends on chorionicity. This is crucial: the same interval for all twins leads to late diagnosis of complications in monochorionic pregnancies.

At each serial examination, growth, amniotic fluid, and signs of complications specific to the chorionicity are assessed. For monochorionic twins, such frequency is needed not only for IUGR but also for timely detection of twin-to-twin transfusion syndrome and anemia-polycythemia.

Monitoring When Suspecting Selective IUGR

With discordance ≥25%, low weight of one twin, or pathological PI of the umbilical artery of the smaller twin, the pregnancy should be shifted from routine screening to monitoring of complicated twins. For monochorionic selective IUGR, ISUOG specifies Doppler assessment of the umbilical artery, middle cerebral artery, and ductus venosus at least weekly, and biometry every 2 weeks.

In dichorionic twins, selective IUGR is managed according to the logic of placental insufficiency in one twin: serial biometry, dynamics of EFW and abdominal circumference, Doppler of the umbilical artery, middle cerebral artery, and ductus venosus according to the clinical situation. The frequency of monitoring is determined by the severity of IUGR, gestational age, and Doppler changes, but should not replace referral to an expert center in cases of significant discordance.

Interpretation of Doppler: Common Mistakes

• Not indicating chorionicity. Without chorionicity, the typification of selective IUGR loses meaning: types I–III pertain to monochorionic twins.
• Calculating the difference from the smaller twin. ISUOG uses the calculation from the weight of the larger twin; otherwise, the percentage of discordance will be overestimated.
• Limiting to one umbilical artery. In complicated twins, MCA and ductus venosus are needed, especially before deciding on management.
• Not distinguishing between persistent and intermittent AREDF. Type II and type III have different mechanisms and clinical interpretations.

Formulating the Conclusion

The optimal formulation should be reproducible: 'Monochorionic diamniotic twins. Twin A: EFW … percentile. Twin B: EFW … percentile. Discordance in EFW …%. In the smaller twin: blood flow in the umbilical artery …; MCA …; ductus venosus …. Ultrasound findings correspond to selective IUGR, type …'. For dichorionic twins, types I–III are not indicated, but the criteria for IUGR and Doppler severity are described.

This format facilitates routing: with ≥25% discordance, pathological PI of the umbilical artery, or absent/reversed end-diastolic flow, the pregnancy should be discussed in a fetal medicine setting where expert Doppler, dynamic monitoring, and timing of delivery are available.