RATIONALE: Hyrtl anastomosis is an intra-arterial shunt between umbilical arteries, which exerts a protective effect by balancing arterial pressure in singleton pregnancies. Its absence may be associated with adverse outcomes such as fetal growth restriction. However, its role in monochorionic twin pregnancies remains unclear. This study aims to explore the clinical significance of the absence of Hyrtl anastomosis in monochorionic twin pregnancies through placental perfusion analysis. PATIENT CONCERNS: Case 1: 32-year-old primigravida with natural conception, diagnosed with monochorionic diamniotic (MCDA) twin pregnancy at 12 weeks' gestation. Fetal growth discrepancy was found at 24 weeks; case 2: 30-year-old G2P0 woman with an in vitro fertilization-conceived MCDA twin pregnancy. INTERVENTIONS: Biweekly regular prenatal examinations throughout pregnancy, serial ultrasound monitoring of fetal growth and blood flow (umbilical artery, middle cerebral artery, ductus venosus). The patient in case 1 received an elective cesarean section at 36 + 5 weeks' gestation, while the patient in case 2 underwent the surgery at 37 weeks' gestation. DIAGNOSES: Case 1:type I selective intrauterine growth restriction; case 2: uncomplicated MCDA twin pregnancy. OUTCOMES: Placental perfusion confirmed the absence of Hyrtl anastomosis in both cases (the larger fetus in case 1, and the smaller fetus in case 2). The newborns had good Apgar scores, and the short-term and medium-term prognoses were excellent. LESSONS: The absence of Hyrtl anastomosis may be a protective factor in monochorionic twin pregnancies, which can offset the adverse effects caused by uneven placental share and vascular communicating anastomoses, thereby improving fetal prognosis.