Norman , J E , Marlow , N , Messow , C-M , Shennan , A , Bennett , P R , Thornton , S , Robson , S C , McConnachie , A , Petrou , S , Sebire , N J , Lavender , T , Whyte , S , Norrie , J & OPPTIMUM study group 2016 , ' Vaginal progesterone prophylaxis for preterm birth (the OPPTIMUM study) : a multicentre, randomised, double-blind trial ' The Lancet , vol 387 , no. 10033 , pp. 2106-2116 . DOI: 10.1016/S0140-6736(16)00350-0