Association of maternal prenatal copper concentration with gestational duration and preterm birth: a multicountry meta-analysis by Monangi, NK, Xu, H, Fan, Y-M, Khanam, R, Khan, W, Deb, S, Pervin, J, Price, JT, Kaur, L, Villar, J, McGready, R, Barros, FC, Victora, CG, Munim, S, Papageorghiou, A, Ochieng, R, Craik, R, Barososio, HC, Berkley, JA, Carvalho, M, Ismail, LC, Lambert, A, Norris, SA, Tshivuila-Matela, C, Nosten, F, Uauy, R, Bhutta, Z, Kennedy, S