Standardizing Abortion Research Outcomes (STAR): a protocol for developing, disseminating, and implementing a core outcome set for medical and surgical abortion by Duffy, J, Whitehouse, K, Kim, C, Ganatra, B, Blum, J, Brahmi, D, Creinin, M, DePiñeres, T, Gemzell-Danielsson, K, Grossman, D, Winikoff, B, Gülmezoglu, A