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Objective: To assess the value of measuring cervical resistance index (CRI) as an aid to selecting patients with a history of spontaneous mid-trimester miscarriage for cervical cerclage in subsequent pregnancies., Study Design: An observational study of 175 patients with a history of one or more spontaneous mid-trimester losses and 123 non-pregnant women who had CRI measurements performed while undergoing routine gynaecological surgery. Those women whose CRI indicated an incompetent cervix were recommended for cervical cerclage in future pregnancies while women with a normal CRI were recommended for conservative management without cerclage., Results: The median CRI in the 123 control women was 38.26 N while the median CRI in the study group was 17.00 N. In 62 of the 175 study women (35%) the CRI findings were at variance with the history of previous mid-trimester loss; 30 (16.6%) were deemed competent on CRI whereas the history suggested incompetence and 32 (18.4%) were incompetent on CRI while the history suggested that the cervix should be competent. The 175 study women had had 486 previous pregnancies with a successful outcome in 27.4% of the pregnancies. Ninety-four patients have now had 148 pregnancies with a successful outcome in 75.8% of the pregnancies., Conclusions: Non-pregnant women with a history of spontaneous mid-trimester miscarriage have a significantly lower cervical resistance index than parous women who have not suffered mid-trimester loss. In 35% of patients the CRI was at variance with the history of the previous loss. CRI may be a useful technique to aid the diagnosis of cervical weakness allowing a rational selection for treatment with prophylactic cervical cerclage.