Monochorionic Twins: Ultrasound Monitoring Schedule According to ISUOG 2025

What Changes with Monochorionicity

Monochorionic twins are not just two pregnancies in one uterus, but a single placental hemodynamic system with vascular anastomoses. Therefore, the standard monitoring interval for dichorionic twins is insufficient here: complications can develop between scheduled screenings. In the updated document ISUOG 2025 Updated Practice Guidelines: role of ultrasound in twin pregnancy, the basic principle is formulated practically: first, accurately determine chorionicity, then manage the monochorionic pregnancy with a biweekly ultrasound schedule for early detection of TTTS, TAPS, and selective growth restriction.

Start: Determine Chorionicity Before 14 Weeks

The optimal window for determining chorionicity is 11+0–13+6 weeks. The physician should document the number of gestational sacs, amnions, placental masses, the thickness of the intertwin membrane, and ultrasound signs at its attachment to the placenta. The lambda sign is typical for dichorionicity, while the T-sign is indicative of monochorionicity. If chorionicity is unclear after the first trimester, it is safer to manage the pregnancy as monochorionic until proven otherwise, as the cost of missing TTTS or TAPS is higher than the cost of more frequent monitoring.

Basic Ultrasound Monitoring Schedule for MCDA

For uncomplicated monochorionic diamniotic twins, ISUOG recommends starting specialized ultrasound monitoring at 16 weeks and repeating it every 2 weeks. This interval is primarily chosen for the early detection of twin-to-twin transfusion syndrome, which usually manifests in the second trimester and requires rapid referral to a fetal therapy center.

Minimum Protocol for Each Visit

In the report, it is advisable to separately describe fetus A and fetus B, using a consistent method of identification: position, side of the uterus, relation to the internal os, gender if possible, placental and umbilical cord features. At each biweekly visit for MCDA, assess the heartbeats of both fetuses, the maximum vertical pocket of amniotic fluid in each amnion, bladder filling, biometry and estimated weight, growth discordance, and Doppler indices according to the clinical situation. For TAPS screening after 20 weeks, the peak systolic velocity in the middle cerebral artery of both fetuses is crucial.

Key ISUOG Thresholds for Practice

TTTS: How Not to Miss It

Twin-to-twin transfusion syndrome in MCDA is diagnosed not by size difference, but by a combination of pronounced amniotic fluid imbalance: one fetus with DVP <2 cm, the other with polyhydramnios according to gestational threshold. Additionally, assess the donor's bladder, Doppler abnormalities, hydrops, and fetal viability. These signs correspond to Quintero's clinical staging: from stage I with a visible donor bladder to stage V with the death of one or both fetuses. If TTTS is suspected at gestational ages potentially suitable for fetoscopic laser coagulation of anastomoses, the report should directly recommend urgent referral to a fetal medicine center.

Selective Growth Restriction: What to Include in the Report

In monochorionic twins, selective IUGR is associated not only with placental insufficiency but also with unequal placental territory distribution and anastomoses. ISUOG uses EFW discordance ≥25% as an important risk threshold. The protocol should specify the centile of EFW and abdominal circumference of each fetus, the degree of discordance, fluid volume, and Doppler. For monochorionic selective IUGR, classification by blood flow in the umbilical artery of the smaller fetus is crucial: type I — positive end-diastolic flow, type II — persistently absent or reversed, type III — intermittently absent or reversed. Types II and III require expert monitoring due to a higher risk of sudden deterioration.

TAPS: Why MCA-PSV is Needed

Twin anemia-polycythemia sequence can occur spontaneously or after TTTS treatment. Unlike TTTS, TAPS may not have the classic polyhydramnios/oligohydramnios pair, so a single DVP measurement is insufficient. Screening is based on comparing MCA-PSV in both fetuses: increased in the presumed anemic fetus and decreased in the polycythemic fetus. Threshold criteria are MCA-PSV >1.5 MoM in the donor and <1.0 MoM in the recipient or an intertwin difference >0.5 MoM. It is important to report MoM, not just absolute velocity, as the indicator depends on gestational age.

Monochorionic Monoamniotic Twins

MCMA is a separate high-risk group. There is no intertwin membrane here, so TTTS criteria based on separate amniotic pockets are not applicable, but the risk of cord entanglement and sudden events is higher. Ultrasound should confirm the absence of a membrane, exclude conjoined twins, assess anatomy, growth, and Doppler of each fetus. The frequency of monitoring and timing of delivery are determined by a center experienced in managing MCMA; however, ISUOG principles remain the same: early accurate diagnosis of amnionicity, regular monitoring of both fetuses, and a low threshold for referral to a specialized center.

Practical Referral Template

The ultrasound diagnostician should not only describe the findings but also indicate the risk. The wording can be brief: MCDA, gestational age, both fetuses alive, DVP right/left, bladders visualized or not, EFW and discordance, UA/MCA-PSV as indicated, no signs of TTTS/TAPS or suspicion present. If DVP <2 cm in one fetus and polyhydramnios in the other, MCA-PSV pattern of TAPS, EFW discordance ≥25%, pathological blood flow in the umbilical artery of the smaller fetus, or the death of one fetus, the report should conclude with a recommendation for urgent consultation with a fetal medicine specialist.