Algorithm for ET/PV in pregnancy. Footnote Risk factors for complications in pregnancy: Previous hemorrhage or venous or arterial thrombosis in mother (whether pregnant or not); Previous pregnancy complication that may have been caused by ET or PV: Unexplained recurrent first trimester loss (three unexplained first trimester losses); Intrauterine growth restriction (birthweight <5th centile for gestation); Intrauterine death or still birth (with no obvious other cause, evidence of placental dysfunction, and growth-restricted fetus); Severe preeclampsia (necessitating preterm delivery <34 weeks) or development of any such complication in the index pregnancy; Placental abruption; Significant antepartum or postpartum hemorrhage (requiring red-cell transfusion); Marked sustained rise in platelet count rising to above 1500 × 109/L