Adnexal tumors during pregnancy are rare, with a prevalence ranging from 0.05% to 3%, and in most cases, they are benign. Struma ovarii, a monodermal teratoma, consists of over 50% thyroid tissue and accounts for 2.7% of ovarian teratomas. It typically affects women aged 40–60 and is exceptionally rare during pregnancy. Diagnosis is often only established after surgical intervention and histological examination. We present the case of a 39-year-old pregnant woman (gravida 2, para 1) at 19 weeks of gestation who presented with acute lower abdominal pain. At her first visit at 11 weeks, ultrasound revealed a 12 cm multilocular left adnexal mass. Initial conservative management was followed by emergency laparoscopy due to suspected ovarian torsion, resulting in a left oophorectomy. Histopathology confirmed struma ovarii. Thyroid function tests (TSH, FT4) remained within normal limits throughout pregnancy. The pregnancy continued without further complications, culminating in a spontaneous vaginal delivery at 40 + 4 weeks of a healthy female infant weighing 3800 g. Due to the rarity of this condition, treatment guidelines remain undefined, with management decisions relying on limited case reports and clinical judgment. This report highlights the importance of detailed evaluation and individualized management in such uncommon presentations during pregnancy.

Twin anemia–polycythemia sequence (TAPS) in monochorionic twin pregnancies is a potentially serious complication caused by unidirectional vascular anastomoses in the placenta, resulting in one anemic donor twin and one polycythemic recipient twin. Diagnosis of this condition is achieved through Doppler ultrasound assessment of the difference between the MoM of the peak systolic velocity of the middle cerebral artery between the twins, establishing the diagnosis with a delta value >0.5 MoM. Management of this situation is individualized and may include intrauterine transfusions, intrauterine laser treatment, and expectant management through ultrasound monitoring of both fetuses to prevent complications. In severe cases, pregnancy termination may be necessary. It is essential that these pregnancies are managed by a multidisciplinary team of professionals, including obstetricians specialized in fetal medicine and neonatologists, to ensure the best possible outcome for both the mother and the fetuses. Early detection and treatment are crucial in the management of pregnancies complicated by twin anemia–polycythemia sequence. The main objective of this article is to conduct a review of the existing literature on the anemia–polycythemia sequence in monochorionic pregnancies, emphasizing the exceptional nature of the presented case due to its spontaneous occurrence, which has a very low prevalence compared to post-laser TAPS cases. It also discusses the different treatment options, highlighting the importance of expectant management and individualization in each case.