Cervical changes in twin pregnancies observed by transvaginal ultrasound during the latter half of pregnancy: a longitudinal, observational study.

Objective: To determine what constitutes normal cervical changes in twin gestations during the latter half of pregnancy, and whether there are differences in the pattern of cervical change between women expecting twins giving birth at term (> or = 36 completed gestational weeks, GW) and preterm.

Methods: Twenty women (12 nulliparous, eight parous) expecting twins were examined with transvaginal ultrasound every week from 24 GW until delivery. Cervical length and width were measured, the inner cervical os was assessed as being closed or open, and any dynamic cervical changes were noted. Examination results were unavailable to the staff involved in the care of the women.

Results: Eight women (40%) delivered preterm spontaneously at 32-35 GW. Cervical length remained unchanged ( approximately 10 mm) in one woman, who delivered preterm. It decreased in the remaining 19 women. The median shortening rate for women manifesting a continuous shortening of the cervix was 2.9 (0.8-5.2) mm/week in women giving birth preterm vs. 1.8 (0.8-2.4) mm/week in those who gave birth at term (P = 0.08). Median cervical length at the first examination was similar in women who delivered preterm and at term (39 vs. 41 mm), but at 32 GW the cervix was shorter in women who delivered preterm (median, 18 vs. 31 mm, P = 0.02). Cervical width remained unchanged in 63% (5/8) of the women who delivered preterm vs. 8% (1/12) of those who gave birth at term (P = 0.02). The cervix became wider in the remaining women. Median cervical width at the first examination was similar in women who delivered preterm and at term (38 vs. 38 mm), but in women who delivered preterm the cervix was thinner at 0-6 days before spontaneous start of labor (median, 40 vs. 48 mm, P = 0.07). An opening of the inner cervical os was seen < or = 27 GW in 88% (7/8) of the women who delivered preterm vs. 17% (2/12) of those who delivered at term (P = 0.006), the corresponding figures for dynamic changes being 63% (5/8) vs. 17% (2/12) (P = 0.06). Dynamic changes were seen in a greater proportion of the examinations in women who delivered preterm (median, 25% vs. 4%, P = 0.07).

Conclusions: The pattern of cervical changes from 24 GW to delivery differs between twin pregnancies delivering preterm (at 32-35 GW) and at term (> or = 36 GW). In twin pregnancies delivered preterm cervical shortening is more rapid, the cervix does not broaden to the same extent as in twins delivered at term, an open inner cervical os and dynamic cervical changes are seen earlier in gestation, and dynamic cervical changes are seen more often. Cervical changes in women with twins who give birth as early as at 24-31 GW may be different.