Fig. 1

Flow Chart of Subject Selection. TTTS = Twin Twin Transfusion Syndrome. ^a some infants excluded for more than one reason. ^b the co-twin was excluded because they had been coded with TTTS, but the twin that remains in the database was not coded as having TTTS. ^c the co-twin was excluded for having twin type coded as monoamniotic, but the twin that remains in the database was not coded as having this twin type. ^d exact numbers cannot be reported in order to preserve confidentiality of the research subjects.^e of the infants without a co-twin in the dataset, these ones have a code corresponding to continuing pregnancy after spontaneous abortion of one fetus or more, after selective fetal reduction, or after intrauterine fetal demise – in other words, ICD10CA codes of O31.11×, O31.12×, or O31.2; there were no instances of O31.0 (papyraceous fetus)