Cervical length on transvaginal ultrasound is shorter and the risk of preterm labour higher than in singletons, so the cut off for identifying those at risk or preterm delivery before 32-34 weeks is set higher at 20-25 mm . Cervical length of 20 mm is found in 8% of twins at 23 weeks but 40% of those delivering spontaneously <32 weeks. Meta-analysis of studies in asymptomatic twin pregnancies shows that a cervical length of <25 mm at 20-24 weeks increases the pre-test probability of delivery before
34 weeks from 19 to 48%, whereas a negative test reduces it only slightly to 14% . Although useful for identifying those at risk, screening remains controversial because of the lack of evidence that cerclage improves outcome in twin pregnancies with a short cervix ; indeed, the only randomized trial showed a doubling in preterm birth.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.