Methylergometrine (product containing ergot alkaloids) has a tendency to produce vaso-constriction. When given intravenously or intramuscularly (usually during the third stage of labour) cause sudden sustained contraction of the uterus. This uterine contraction causes a sudden addition of about 500 ml of blood into the maternal circulation thus increasing the left aerial pressure upto 10 mmHg. This contributes to pulmonary edema leading to right hea failure. Ref: Manual Of Obstetrics (2Nd Edition) By Daftary, Page 129 - 130; Textbook of Obstetrics By Padubidri & Anand, V. Padubidri, Ela Anand, Pages 144 ; Pharmacology and Pharmacotherapeutics (New Revised 21 St Ed.) By R. S. Satoskar, S D Bhandarkar & nirmala N. Rege, Pages 624-627; Meyler's Side Effects of Endocrine and Metabolic Drugs By J. K. Aronson, Page 116