Maternal and fetal factors associated with stillbirth in singleton pregnancies in 13 hospitals across six states in India: a prospective cohort study per Boo, YY, Bora, AK, Chhabra, S, Coudhury, SS, Deka, G, Kakoty, S, Kumar, P, Mahanta, P, Minz, B, Rani, A, Rao, S, Roy, I, Solomi V, C, Verma, A, Zahir, F, Deka, R, Kurinczuk, JJ, Nair, M