Study of Pregnancy in Patients With Myeloproliferative Neoplasms

Inclusion Criteria: * Patients with a diagnosis of chronic phase essential thrombocythemia (ET), polycythemia vera (PV), prefibrotic or overt primary myelofibrosis (PMF), post-ET myelofibrosis (MF), post-PV MF, or myeloproliferative neoplasm (MPN) - unclassifiable according to the 2022 International Consensus Criteria. Patients who have not had a diagnostic bone marrow biopsy may participate if there is a driver mutation and all other diagnostic criteria are met. * Patients with a confirmed pregnancy through Beta Human Chorionic Gonadotropin (BHCG) testing of the urine or blood and/or dating ultrasound. Patients may be eligible to participate at any stage of pregnancy. * Patients must have the ability to participate in study procedures. * Participants must provide consent to participate in this study. Exclusion Criteria: * Patients with a diagnosis of chronic myeloid leukemia, myelodysplastic syndrome (MDS), MDS/MPN overlap syndrome, or other myeloid malignancy. * Peripheral blood or marrow blasts \>/= 10%. * Patients with concurrent disease that in the opinion of the investigator may be adversely impact the safety of the patient or impair assessment of study results, including but not limited to: * Uncontrolled autoimmune diseases (e.g., hepatitis, immune thrombocytopenic purpura \[ITP\], scleroderma, autoimmune arthritis) * Active, uncontrolled infection (e.g., HIV, hepatitis B, hepatitis C) * Second malignancy requiring concurrent treatment, except basal cell carcinoma or squamous cell carcinoma or cervical carcinoma in situ. * Patients requiring a legally authorized representative.