There is now good evidence that transvaginal sonographic measurement of cervical length can be used to predict the risk of preterm labour in both low- and high-risk pregnancies and in women who are symptomatic. Transabdominal measurement of cervical length is unreliable because of the need for a full bladder which may compress the cervix leading to an overestimate of its length and because it is more difficult to obtain adequate views of the cervix with this technique. Two strategies are currently in common use - either serial measurement of cervical length throughout the second and early third trimester of pregnancy or a single measurement of cervical length usually at the time of the routine ultrasound between 18 and 22 weeks. At any given gestational age there is a direct relationship between cervical length and the risk of preterm delivery. So, for example, a cervical length of 15 mm or less at 20-24 weeks predicts a 50% risk of preterm delivery prior to 34 weeks in a low-risk population. In multiple pregnancies the risk of preterm labour is higher, at any given cervical length than in a singleton pregnancy with the same cervical length. A large number of studies have examined the relationship between gestational age, cervical length and the risk of preterm delivery (see table and review by Honest et al. 2002). It appears that it is absolute cervical length rather than the presence or absence of funnelling which is the principle predictor of spontaneous preterm birth. If a screening strategy using a single ultrasound measurement of cervical length is used, then assessment between 21 and 24 weeks gestation appears to be better than assessment prior to 20 weeks gestation in predicting the risk of preterm labour. It is arguable, however, that identification of a risk of preterm labour as late as 23 weeks may be too late for any potential prophylactic therapies to be effective. Serial measurement of cervical length is more costly but appears to be superior to a single measurement in assessing the risk of preterm delivery.
On the continent of Europe it is common practice to perform a vaginal assessment of cervical length at each antenatal consultation. However, multicentre trials have shown that this policy is of no benefit in predicting the risk of preterm delivery.
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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.