01 Confirmation of Gestational Age in Twin Pregnancy

1. The period between 11 and 13+6 weeks of gestation, when the crown-rump length (CRL) is 45-84mm, is the best time to estimate the gestational age of twin pregnancy (Recommendation grade: D).
2. For naturally conceived twin pregnancies, the larger CRL of the two fetuses should be used to estimate gestational age (Recommendation grade: C).
3. For twin pregnancies after in vitro fertilization, the oocyte retrieval date or embryo fertilization time should be used to estimate gestational age (Evidence level: 2+).
4. If the pregnant woman seeks medical advice after 14 weeks of gestation, the larger head circumference of the two fetuses should be used for estimation [2] (Evidence level: 2+).

02 Determination of Chorionicity and Amnionicity in Twin Pregnancy

1. Chorionicity of twin pregnancy should be determined before 13+6 weeks of gestation by assessing the thickness and morphology of the inter-twin membrane at the placental insertion site (lambda sign or T sign, Figures 1 and 2) and the number of placentas [2]. Ultrasound images showing chorionicity-related features should be saved for reference (Recommendation grade: D). For pregnant women who present after 14 weeks of gestation, chorionicity can still be determined by identifying the above ultrasound signs (especially counting the number of membrane layers) and observing fetal gender.

1. If chorionicity cannot be determined by transabdominal or transvaginal ultrasound, the patient should be referred to a tertiary referral center for confirmation (Good practice point). If chorionicity remains unclear, management as a monochorionic twin pregnancy is recommended [2] (Evidence level: 3).
2. Once chorionicity is confirmed, amnionicity should also be determined and documented. All cases of monochorionic monoamniotic twin pregnancy should be referred to a tertiary center with specialized management experience for evaluation [2] (Good practice point).

03 Twin Labeling

04 Ultrasound Monitoring of Twin Pregnancy

1. For uncomplicated dichorionic twin pregnancies, first-trimester ultrasound, detailed second-trimester ultrasound, and subsequent ultrasound examinations every 4 weeks should be performed [2] (Figure 3). For complicated dichorionic twin pregnancies, more frequent ultrasound examinations should be conducted according to the specific condition and severity of the disease (Good practice point).

1. For uncomplicated monochorionic twin pregnancies, in addition to first-trimester ultrasound, ultrasound examinations every 2 weeks starting from 16 weeks of gestation are recommended to detect twin-to-twin transfusion syndrome (TTTS) and twin anemia-polycythemia sequence (TAPS) in a timely manner [4-5] (Figure 4). For complicated monochorionic twin pregnancies, more frequent ultrasound examinations should be performed according to the specific condition and severity of the disease (Recommendation grade: C).

1. Each ultrasound assessment of twin pregnancy should include the following items: biometric parameters of both fetuses, amniotic fluid volume, and umbilical artery Doppler (starting from 20 weeks of gestation). When there is fetal growth discordance between the two fetuses, fetal weight should be re-estimated and recorded at each ultrasound examination starting from 20 weeks of gestation. For monochorionic twin pregnancies, middle cerebral artery peak systolic velocity (MCA-PSV) should be recorded after 20 weeks of gestation to diagnose twin anemia-polycythemia sequence. In monochorionic diamniotic twin pregnancies, the maximum vertical pocket (MVP) of amniotic fluid should be evaluated and recorded at each ultrasound examination to monitor TTTS. Cervical length measurement during the second-trimester structural scan can be used to predict the risk of preterm birth (Evidence levels: 2+, 2++).

05 Screening for Chromosomal Abnormalities in Twin Pregnancy

1. First-trimester combined screening for trisomy 21 can be performed using nuchal translucency (NT) thickness, free β-human chorionic gonadotropin (β-hCG), and pregnancy-associated plasma protein-A (PAPP-A) levels. Screening can also be based solely on maternal age combined with fetal NT value [2] (Recommendation grade: B).
2. In cases of vanishing twin, if a measurable embryo is present, NT value combined with maternal age should be used to assess the risk [6] (Recommendation grade: B).
3. The detection rate of non-invasive prenatal testing (NIPT) for trisomy 21 in twin pregnancy is lower than that in singleton pregnancy, but the supporting data are still limited (Recommendation grade: B).

06 Invasive Prenatal Diagnosis in Twin Pregnancy

1. Chorionic villus sampling (CVS) can be performed earlier than amniocentesis, so CVS is the preferred method for dichorionic twin pregnancies (Recommendation grade: D).
2. For monochorionic twin pregnancies: if the two fetuses have consistent growth and normal structure, only one amniotic cavity needs to be sampled; if rare asymmetric chromosomal abnormalities are suspected in the two fetuses, separate amniocentesis of both amniotic cavities should be performed. However, the complexity of selective fetal reduction should be fully studied before the procedure, and the necessity of the operation should be determined [7] (Evidence level: 3).
3. Significance of inconsistent NT values or CRL in twin pregnancy: Due to the higher risk of fetal abnormalities when the CRL difference between the two fetuses is ≥10% or the NT value difference is ≥20% [8-10], such cases should be discussed with fetal medicine experts (Recommendation grade: B), and detailed ultrasound examination and chromosomal karyotyping should be performed for these cases. CRL discordance at 7-9+6 weeks of gestation is a predictor of the risk of single fetal death in the first trimester [11] (Evidence level: 2++).

07 Ultrasound Screening for Structural Abnormalities in Twin Pregnancy

1. Major fetal malformations should be evaluated during the first-trimester ultrasound examination [12] and re-evaluated during the routine second-trimester ultrasound (malformation exclusion) examination around 20 weeks of gestation (18-22 weeks) [2, 13] (Good practice point).
2. Fetal cardiac screening and assessment for monochorionic twin pregnancies should be performed in accordance with ISUOG guidelines (Good practice point).
3. Pregnant women should be informed of the potential limitations of ultrasound in detecting different types of malformations.

08 Management of Fetal Growth Discordance Caused by Fetal Malformation

09 Selective Fetal Reduction in Twin Pregnancy

1. For dichorionic twin pregnancies, selective fetal reduction is best performed in the first trimester under ultrasound guidance by intracardiac or intraumbilical injection of potassium chloride or lidocaine into the abnormal fetus (Recommendation grade: B).
2. If diagnosed in the second trimester, pregnant women may choose late selective fetal reduction in the third trimester within the scope permitted by law (Good practice point).
3. For monochorionic twin pregnancies, selective fetal reduction can be performed by umbilical cord occlusion, fetoscopic laser ablation, or radiofrequency ablation [14-15] (Recommendation grade: B).

10 Monitoring of Preterm Birth Risk in Twin Pregnancy

11 Diagnosis and Management of Twin Pregnancy Complicated with Selective Fetal Growth Restriction (sFGR)

(I) Diagnosis of Twin Pregnancy Complicated with Selective Fetal Growth Restriction

1. The traditional definition of selective fetal growth restriction (sFGR) is that the weight of either fetus in a twin pregnancy is below the 10th percentile for gestational age, accompanied by a weight difference of >25% between the two fetuses [18-19] (Good practice point).
2. A weight difference of 20% is the threshold for increased risk of adverse prognosis (Recommendation grade: B).
3. Once sFGR is diagnosed, the cause should be identified as early as possible, including ultrasound malformation screening, virological testing, and chromosomal examination; sFGR in monochorionic twin pregnancies is mainly caused by uneven placental share and vascular distribution [20] (Evidence level: 3).
4. sFGR in monochorionic twin pregnancies is classified based on umbilical artery end-diastolic flow (UA-EDF) patterns (Figures 5-7) [21] (Good practice point).

(II) Management of Twin Pregnancy Complicated with Selective Fetal Growth Restriction

1. It is recommended to evaluate the growth of both fetuses in twin pregnancies by combining the measurement parameters of fetal head, abdomen, and femur (Recommendation grade: B).
2. If the weight difference between the two fetuses is ≥25%, referral to a tertiary fetal medicine center for evaluation is recommended [2] (Good practice point).
3. Dichorionic twin pregnancies complicated with sFGR should be followed up according to the singleton fetal growth restriction model. Depending on the severity, fetal Doppler examinations every 2 weeks are recommended (Good practice point).
4. The evidence guiding the management of monochorionic twin pregnancies complicated with sFGR is limited, and fetal Doppler examinations at least once a week are recommended (Good practice point).
5. If there is a significant risk of single fetal intrauterine death before 26 weeks of gestation, selective fetal reduction may be considered (Recommendation grade: D). Such cases should be managed in coordination with a tertiary fetal medicine center [22] (Evidence level: 2-).

(III) Management of the Surviving Fetus After Single Fetal Intrauterine Death in Twin Pregnancy

1. Pregnant women with single fetal intrauterine death in twin pregnancy should be referred to a tertiary medical center with relevant professional knowledge (Good practice point).
2. Fetal Doppler examinations, including middle cerebral artery peak systolic velocity (MCA-PSV), should be performed to detect signs of anemia in the surviving fetus. Conservative treatment (continuation of pregnancy) is usually the most appropriate management. If term, delivery should be terminated immediately; if preterm, conservative treatment is recommended. In the short term, fetal heart rate monitoring and MCA Doppler should be used to assess the surviving fetus for signs of progressive injury and anemia [23]. If conservative treatment is chosen, fetal biometric measurements and umbilical artery and MCA Doppler assessments should be performed every 2-4 weeks, and delivery should be considered at 34-36 weeks of gestation with antenatal corticosteroid administration for fetal lung maturation. Imaging examination of the surviving fetus should be performed approximately 4-6 weeks after the occurrence of single fetal intrauterine death. If there is strong evidence of severe neurological damage in the surviving fetus, termination of pregnancy in the third trimester may be considered. Evaluation of neurological development of the surviving fetus is recommended at 2 years of age.

12 Unique Complications of Monochorionic Twin Pregnancy

(I) Twin-to-Twin Transfusion Syndrome (TTTS)

1. Monochorionic twin pregnancies are at risk of developing TTTS when there is unequal hemodynamics and uneven amniotic fluid distribution between the two fetuses [24-27]. The diagnosis of TTTS requires significant imbalance in amniotic fluid volume between the two fetuses: the donor fetus has a maximum vertical pocket (MVP) of amniotic fluid <2cm (oligohydramnios), while the recipient fetus has an MVP >8cm (polyhydramnios). The European criteria for polyhydramnios are MVP ≥8cm before 20 weeks of gestation or ≥10cm after 20 weeks of gestation. Amniotic fluid imbalance is the most common manifestation but not a mandatory diagnostic criterion. Although the Quintero staging system cannot always accurately predict the prognosis or subsequent progression of TTTS, it remains the preferred staging system [24-25] (Table 2, Good practice point).

1. Monitoring of TTTS: For monochorionic twin pregnancies, ultrasound monitoring for TTTS is recommended starting from 16 weeks of gestation, with re-examinations every 2 weeks (Good practice point). During each ultrasound scan, the operator should note and record amniotic membrane folding and measure the MVP of each fetus. If there is a significant difference in MVP between the two fetuses or intra-amniotic membrane folding, the frequency of ultrasound examinations should be increased.
2. Monochorionic twin pregnancies with isolated amniotic fluid imbalance should be followed up weekly to rule out the development of TTTS (Good practice point).
3. Treatment of TTTS: (1) Laser ablation is the treatment of choice for TTTS of Quintero stage II or higher [27] (Recommendation grade: A). (2) For Quintero stage I, conservative management with close surveillance or laser ablation may be considered (Recommendation grade: B). (3) If laser ablation is not available, serial amnioreduction after 26 weeks of gestation is an alternative treatment [27] (Recommendation grade: A). (4) Usually, ultrasound assessments should be performed weekly within 2 weeks after treatment, then reduced to every other week based on clinical conditions (Good practice point). Each ultrasound scan should assess the MVP of both fetuses, fetal biometric measurements (every 2 weeks), and umbilical artery, MCA (PSV), and ductus venosus (DV) Doppler. (5) If single fetal intrauterine death occurs (after laser treatment), brain imaging of the surviving fetus should be performed 4-6 weeks later, and neurological development should be evaluated at 2 years of age (Good practice point).

(II) Twin Anemia-Polycythemia Sequence (TAPS)

1. TAPS is generally considered to be caused by slow transfusion from the donor fetus to the recipient fetus through small arteriovenous anastomoses (<1mm), resulting in extreme postnatal hemoglobin (HGB/Hb) discrepancy between the two fetuses (Evidence level: 3).
2. Discordant abnormal middle cerebral artery Doppler is the basis for the prenatal diagnosis of TAPS (Recommendation grade: D). This includes MCA-PSV >1.5 multiples of the median (MoM) in the donor fetus (indicating fetal anemia) and MCA-PSV <1.0 MoM in the recipient fetus (indicating polycythemia). Other ultrasound findings include unequal placental echo and uneven thickness (thickened and hyperechoic placenta in the donor fetus, thin and anechoic placenta in the recipient fetus). The polycythemic fetus may show a "starry sky" appearance on liver ultrasound due to decreased hepatic parenchymal echo and enhanced portal vein wall echo (Evidence level: 3).
3. The postnatal diagnosis of TAPS is based on chronic anemia in the donor fetus and polycythemia in the recipient fetus. Diagnostic criteria include a hemoglobin concentration difference of >8g/dL between the two fetuses, accompanied by a reticulocyte ratio (donor fetus/recipient fetus) >1.7 or small placental vascular anastomoses (diameter <1mm) [28-29] (Evidence level: 3).
4. TAPS is staged according to the severity of prenatal and postnatal conditions [28-29] (Table 3, Evidence level: 3).