Measurement of cervical lengthIn the measurement of cervical length by transvaginal sonography:

The woman empties her bladder and is then placed in the dorsal lithotomy position.

The ultrasound probe is introduced in the vagina and directed in the anterior fornix. Care is taken to avoid exerting undue pressure on the cervix, which may artificially increase the length.

A sagittal view of the cervix is obtained and the endocervical mucosa (which may be of increased or reduced echogenicity compared to the cervix) is used as a guide to the true position of the internal os, thereby avoiding confusion with the lower segment of the uterus.

The calipers are used to measure the linear distance between the triangular area of echodensity at the external os and the V-shaped notch at the internal os.

Each examination should be performed over a period of 2-3 minutes. In about 1% of cases the cervical length may change due to uterine contractions and in such cases the shortest measurement should be recorded.Repeatability of measurement. Transvaginal measurement of cervical length is highly reproducible and in 95% of occasions the difference between two measurements by the same sonographer or by two different sonographers is about 4 mm or less.

TechniqueThe best approach for measurement of cervical length is by transvaginal sonography.Transabdominal sonography fails to visualise the cervix in a high proportion of cases and in particular those with a short cervix. Furthermore, successful

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visualisation requires a full bladder which falsely increases cervical length.Transperineally the cervix can be examined by placing a curvilinear probe sagitally between the labia majora. The cervix can be visualised adequately by translabial-transperineal sonography in about 80% of patients and the measurements of cervical length obtained by this approach are very similar to those obtained by transvaginal sonography. However, in about 20% of cases the cervix can not be visualised or the the external or internal os is obscured by a translucency that can not be abolished despite vertical movement of the transducer or lateral angulation and rotation.

TechniqueCurved cervixThe cervix is often curved and in these cases the measurement of cervical length taken as a straight line between the internal and external os is inevitably shorter than the measurement taken along the endocervical canal.From the clinical point of view the method of measurement is not important because when the cervix is short it is always straight.Cervical funnelingDilatation of the internal os, observed sonographically as funneling, is no more than a simple reflection of the process of producing cervical shortening that will eventually result in preterm delivery. Almost all women with a short cervix have funneling of the internal os.Women with a long cervix and funneling are not at increased risk of preterm delivery.

Clinical applicationSonographic measurement of cervical length is clinically useful in the following situations:

Prediction of preterm delivery in asymptomatic women. In women with a previous history of preterm birth and in those with uterine abnormalities, such as unicornuate

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uterus, the cervical length should be measured every two weeks at between 14 and 24 weeks of gestation. In women with no such history measurement of cervical length should be carried out at 20-24 weeks.

Prediction of the likelihood of delivery within the subsequent seven days in women presenting with threatened preterm labor.

In pregnancies presenting with polyhydramnios during the 2nd or early 3rd trimester of pregnancy measurement of cervical length is useful in deciding whether to carry out amniodrainage.

In women with planned cesarean section measurement of cervical length at 37 weeks can help in deciding whether to carry out the delivery at 37-38 weeks or delay this until 39-41 weeks.

In women undergoing induction of labor the pre-induction cervical length gives useful prediction of the induction-to-delivery interval and the likelihood of vaginal delivery and cesarean section.

In prolonged pregnancies measurement of cervical length at 41 weeks can predict the likelihood of spontaneous onset of labor and vaginal delivery over the subsequent 10 days.

Preterm deliveryImpact of preterm deliveryAbout 130 million babies are born every year in the world and 4 million of these die in the first four weeks of life, the neonatal period.The main direct causes of neonatal death globally are preterm birth, severe infections and asphyxia.In developed countries preterm delivery is responsible for about half of all neonatal deaths.

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Rate of preterm deliveryPreterm birth rates have not decreased in the last 50 years and in most developed countries continue to rise despite advances in knowledge of the risk factors and mechanisms related to preterm labour.There are essentially two reasons for this failure to reduce preterm birth:

Absence of an effective screening test to identify the women at high-risk.

Lack of an effective intervention to prevent this complication.

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Types of preterm deliveryIn one-third of preterm births the delivery is carried out for medical indications, mainly preeclampsia and fetal growth restriction, and in two-thirds it is spontaneous due to premature onset of labour or preterm pre-labour rupture of membranes.All births before 37 weeks of gestation are defined as preterm and these are subdivided according to the gestation at delivery into:

Extreme (<28 weeks), which occurs in about 0.25% of pregnancies.

Early (28-30 weeks), which occurs in about 0.25% of pregnancies.

Moderate (31-33 weeks), which occurs in about 0.6% of pregnancies.

Mild (34-36 weeks), which occurs in about 3.0% of pregnancies.The risk of fetal death and handicap is mainly increased in the extreme, early and moderate subgroups. The overal

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prevalence of spontaneous delivery before 34 weeks is about 1%.

Screening for preterm deliveryThere are essentially two groups of pregnant women contributing to spontaneous deliveries before 34 weeks:

15% of such deliveries come from the group of women who had a previous late miscarriage or spontaneous preterm delivery. This group constitutes about 3% of the pregnant population. In these women the risk of recurrence is inversely related to the gestational age at the previous preterm delivery.

85% of such deliveries come from the 97% of women who are either in their first pregnancy or their previous pregnancies resulted in deliveries at term.Consequently, any strategy at reducing the rate of preterm delivery that is focused on the subgroup of women with a previous preterm birth would have a very small impact on the overall rate of preterm delivery.

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There are essentially two strategies for identifying the high-risk group among women who are either in their first pregnancy or their previous pregnancies resulted in deliveries at term:

Cervicovaginal fetal fibronectin at 22-24 weeks. Fetal fibronectin is an extracellular matrix glycoprotein produced by amniocytes and by cytotrophoblast. It is localized between chorion and decidua where it acts as a ’glue’ between the pregnancy and the uterus. Fetal fibronectin can be detected in cervicovaginal secretions in all pregnancies with high levels before 22 weeks but low levels (less than 50 ng/mL) at 22-34 weeks. Measurement of fetal fibronectin at 22-24 weeks is useful in predicting pregnancies at increased risk of spontaneous preterm birth. At 22-24 weeks the test is positive in about 4% of women including about 25% of those delivering spontaneously before 34 weeks.

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Cervical length at 22-24 weeks is normally distributed with a mean of 36 mm. The risk of spontaneous preterm delivery is inversely related to cervical length and increases exponentially when the cervical length is less than 15 mm. The length is 15 mm or less in about 1% of women including about 30% of those delivering spontaneously before 34 weeks. In twin and triplet pregnancies there is also an inverse relation between cervical length and risk of spontaneous preterm delivery. However, in such multiple pregnancies the cut-off for exponential increase in risk is 25 mm.

Screening by cervical length and obstetric historyA model combining cervical length and obstetric history provides a better prediction of spontaneous preterm birth

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than either factor alone and the sensitivity of screening improves for increasing degrees of prematurity.For a 5% screen positive rate, the detection rate is about 70% for extreme, 45% for early, 40% for moderate and 15% for mild spontaneous preterm birth. The respective values for a 10% screen positive rate are 80%, 60%, 55% and 30%.Preterm delivery calculatorWhen you obtain the FMF Certificate of competence in cervical assessment you will have access to this calculator which will allow you to assess the risk of spontaneous preterm labor.

Prevention of preterm birthStudies investigating the value of preventative measures have essentially focused in two groups of women:

Women with a previous preterm delivery.

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Women with no previous preterm delivery but found through a screening test in pregnancy to be at increased risk of preterm delivery.

Prevention in women with previous preterm birthsNo benefit in reducing the risk of recurrence has been shown for the following strategies:

Bed rest in hospital or at home is widely recommended for the prevention of preterm birth but there is no scientific evidence to support this practice. Indeed, randomized studies in twin pregnancies showed that bed rest increases the risk of preterm delivery. Bed rest may also have some adverse effects for women, including increased likelihood of venous thrombosis, muscle atrophy and stress.

Betamimetics given prophylactically. Life style interventions, such as decrease in manual

labor, increase in visits to antenatal clinics, psychological support, or diet supplementation with iron, folate, calcium, zinc magnesium, vitamins, or fish oil.

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Prevention in women with previous preterm birthsBenefit in reducing the risk of recurrence has been proven for the following two strategies:

Cervical cerclage reduces the risk of delivery before 34 weeks by about 25%. There are two options in the management of patients with previous preterm births. Firstly, elective cerclage in all women soon after the 11-13 weeks scan demonstrates no major fetal abnormality. Secondly, measurement of cervical length every two weeks and placement of a suture only if cervical length becomes of less than 25 mm. The overall rate of preterm delivery is similar with the two approaches but the second approach is preferable because it reduces the need for cerclage by about 50%.

Progesterone given from 20 to 34 weeks reduces the risk of delivery before 34 weeks by about 25%. Progesterone can be given as a vaginal pessary of the natural product (200 mg) every night or as an intramuscular injection of the synthetic 17 alpha-hydroxyprogesterone carporate (250 mg). Natural progesterone is preferable because of lack of undesirable side effects, such as sleepiness, fatigue and headaches. Additionally, there is some concern that injections of 17-OHP-C may increase the risk of fetal death.

Women with previous preterm birth No benefit from bed rest, prophylactic tocolytics or

lifestyle interventions Progesterone vaginal pessary every night from 20 to 34

weeks reduces preterm delivery rate by 25% Measurement of cervical length every 2 weeks between

14 and 24 weeks and cervical cerclage if the cervix becomes less than 25 mm reduces preterm delivery rate by 25%

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Prevention in women with inflammation or infectionIn women with a positive fetal fibronectin test the risk of preterm delivery is increased but this is not reduced by the prophylactic use of antibiotics.Bacterial vaginosis is an imbalance of vaginal flora caused by a reduction of the normal lactobacillary bacteria, and a heavy overgrowth of mixed anaerobic flora including Gardnerella vaginalis, Mycoplasma hominis and Mobiluncus species. Bacterial vaginosis is present in about 20% of women during pregnancy and the majority of cases are asymptomatic. The risk of preterm delivery is doubled in women with bacterial vaginosis and may be increased by 5-times if the infection is present before 16 weeks. Antibiotic treatment can eradicate bacterial vaginosis in pregnancy but does not reduce the risk of spontaneous preterm delivery. However, there is a need for further investigation as to whether treatment before 16 weeks may reduce the risk of preterm delivery.In women with asymptomatic bacteriuria the risk of preterm delivery and pyelonephritis is increased. Antibiotic treatment reduces the risk of pyelonephritis but does not reduce the risk of spontaneous preterm delivery.

Women with no previous preterm birth but positive screening test

Fetal fibronectin positivity. Antibiotics do not reduce but rather increase the risk of preterm delivery

Bacterial vaginosis. Antibiotics do not reduce the risk of preterm delivery. Perhaps antibiotic treatment is useful if given before 16 weeks

Short cervix at 20-24 weeks. Cervical cerclage may reduce the risk of preterm delivery by 15%. Progesterone vaginal pessary every night from 20 to 34 weeks reduces the risk by about 45%

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Prevention in women with short cervixIn women with a short cervix diagnosed by routine transvaginal sonography at 22-24 weeks two strategies have been attempted to reduce the risk of preterm delivery:

Progesterone, (vaginal administration of 200 mg per night at 24-34 weeks) in women with a cervix of 15 mm or less at 20-24 weeks reduces the rate of spontaneous delivery before 34 weeks by about 45%.

Cervical cerclage in singleton pregnancies with a short cervix and no previous history of preterm delivery reduces the risk of spontaneous delivery before 34 weeks by only 15%.

Randomized studies in twin pregnancies with a short cervix (less than 25 mm) showed that cervical cerclage increases the risk of preterm delivery.

Summary of attempts at preventionIn singleton pregnancies in women with previous preterm birthsthe rate of recurrence is reduced by about 25% either by the prophylactic administration of progesterone or by cervical cerclage.In singleton pregnancies in women with no previous preterm births but a short cervix (15 mm or less) diagnosed by routine sonography at 20-24 weeks the risk of delivery before 34 weeks is very high and this is reduced by about 45% through the prophylactic administration of progesterone.Despite the partial success of treatment there remains a major challenge for further reduction of the rate of preterm delivery in high-risk pregnancies.In twin pregnancies the rate of spontaneous preterm birth before 34 weeks is about 13%, compared to 1% in singletons. Randomized studies in twin pregnancies have shown that:

Bed rest is associated with a significant increase, rather than decrease, in the rate of early preterm birth.

Cervical cerclage in those with a short cervix (less than 25 mm) doubles the risk of early preterm birth.

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Prophylactic administration of progesterone does not reduce the risk of early preterm birth.

Vaginal pessary. The FMF is conducting multicentre studies to determine if in singleton and twin pregnancies the insertion of a vaginal pessary reduces the chances of preterm birth. The pessary, which consists of flexible silicone, is used to support the cervix and change its direction towards the sacrum.Pessary study in singletons / twins

Threatened preterm laborManagementWomen presenting with threatened preterm labor are often treated with:

Hospitalisation in a unit with facilities for neonatal intensive care.

Administration of tocolytics to prevent preterm delivery. Administration of steroids to improve fetal lung maturity.

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However, randomised studies on the use of tocolytics in threatened preterm labour have demonstrated a significant prolongation of pregnancy by about seven days but no significant reduction in the incidence of preterm delivery.Consequently, the primary aim of tocolysis is to achieve prolongation of pregnancy for a couple of days for effective treatment with steroids for fetal lung maturity, rather than prevention of preterm delivery as such.

Management of threatened preterm labor:Distinguish between true and false labor:

Cervical length Cervicovaginal fetal fibronectin

In those with true labor: Hospitalization Tocolytics to delay delivery Steroids to mature fetal lungs

ManagementThe majority of women (more than 80%) presenting to the labor ward with painful and regular uterine contractions at 24–36 weeks of gestation are not in true labor and do not deliver within the subsequent seven days.

In the past there were no reliable ways to distinguish between true and false labor. It could therefore be argued that the high mortality and handicap rates associated with early preterm delivery justify treatment of all patients with threatened preterm labor, because such risks outweigh the economic cost of hospitalisation, the maternal risks associated with tocolytics and the potential fetal risks associated with steroids.

Now there is evidence that in women with threatened preterm labor the rate of delivery within the subsequent seven days is inversely related to cervical length at presentation. The cut-off in cervical length that apparently distinguishes between false and true labor in pregnancies presenting with threatened preterm labor is 15 mm for singletons and 25 mm for twins. Therefore, hospitalisation and the administration of tocolytics and steroids should be reserved for women that are truly in

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labor and such women can be identified by sonographic measurement of cervical length at presentation. If the cervical length is more than 15 mm in singletons or more than 25 mm in twins the risk of delivery within the subsequent seven days is less than 1%.

PolyhydramniosCausesPolyhydramnios in the 2nd or early 3rd trimester is found in about 1 per 200 pregnancies. There are essentially two major causes of polyhydramnios:

Reduced fetal swallowing, which may be due to neuromuscular disorders, facial or neck tumors, gastrointestinal obstruction, compressive pulmonary

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defects, or narrow thoracic cage due to skeletal dysplasias.

Increased fetal urination, which is observed in maternal diabetes, hyperdynamic fetal circulation (due to fetal anemia, fetal and placental tumors, arteriovenous malformations), or twin-to-twin transfusion syndrome. 

ManagementIn the management of pregnancies with polyhydramnios the aims are to:

Diagnose and treat the underlying cause, including glycemic control of maternal diabetes mellitus, thoracoamniotic shunting for fetal pulmonary cysts or pleural effusions, intrauterine blood transfusion for fetal anemia, laser occlusion of placental anastomoses in twin-to-twin transfusion syndrome, or laser occlusion of feeding vessels to fetal or placental tumors.

Reduce the risk of very preterm delivery by repeated amniocenteses and drainage of large volumes of amniotic fluid. However, the procedure itself may precipitate preterm birth and amniocentesis is reserved for the cases where the cervical length is less than 15 mm or serial scans on a weekly basis demonstrate shortening of the cervix to below 25 mm.

An alternative and effective method of treatment is maternal administration of indomethacin; however, this drug may cause fetal ductal constriction, and close monitoring by serial fetal echocardiographic studies is necessary.

Timing of elective cesarean sectionThere is a contradiction in the timing of elective cesarean sections which are traditionally performed at 38 weeks.

Delay beyond 38 weeks will result in many women going into spontaneous labor and consequently an elective

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procedure will be converted into an emergency one with increased maternal mortality and morbidity.

The risk of respiratory morbidity in the neonate is halved with each completed week of gestation between 37 and 41 weeks.Measurement of cervical length at 37 weeks is useful in predicting spontaneous onset of labor and delivery at term. This information can be utilized in individualizing the gestation for elective cesarean section:

In women with cervical length of less than 20 mm there is a 95% chance of spontaneous onset of labor before 40 weeks. In such patients elective cesarean section is best carried out at 37–38 weeks to avoid the increased risk of maternal mortality and morbidity from emergency as compared with elective surgery.

In women with cervical length of more than 30 mm there is a 95% chance that spontaneous onset of labor will not occur before 40 weeks. In such patients elective cesarean section could be delayed until 40 weeks to reduce neonatal respiratory morbidity.

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Post-term pregnancyImplicationsIn 5-10% of women their pregnancies continue beyond 294 days (42 completed weeks) and are described as being post-term. Such pregnancies are at increased risk of birth trauma and asphyxia as well as stillbirth and postnatal death.The risk of pregnancy loss (intrauterine, neonatal and infant death) increases from 1.2 per 1000 ongoing pregnancies at 38 weeks of gestation to 2.4 at 40 weeks and 5.8 at 43 weeks.In the management of post-term pregnancies, induction of labor compared to expectant management, is associated with a substantial reduction in perinatal mortality. However, approximately 20% of women having induction of labor need an emergency cesarean section for delivery because of failed induction, failure to progress in labor or fetal distress. The rate of cesarean

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section is dependent on parity, being about 10% for parous women and 30% for nulliparae.

Induction of laborIn women undergoing induction of labor the pre-induction sonographic measurement of cervical length in addition to maternal characteristics (parity, gestational age, maternal age, and body mass index) provides useful prediction of the induction-to-delivery interval, the likelihood of vaginal delivery within 24 hours and the likelihood of cesarean section.

In parous women the likelihood of cesarean section for failed induction, failure to progress or fetal distress increases with pre-induction cervical length from about 2% for cervix of 1-10 mm to 20% for cervix of more than 30 mm.

In nulliparous women the likelihood of cesarean section for failed induction, failure to progress or fetal distress increases with pre-induction cervical length from

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about 10% for cervix of 1-10 mm to 70% for cervix of more than 30 mm.

Delay of induction to the 42nd weekIn prolonged pregnancy induction of labor is usually carried out during the 41st week. Another approach is to undertake an ultrasound-based assessment during the 41st week to identify previously undiagnosed problems, such as breech presentation that would require cesarean section or oligohydramnios and fetal growth restriction for which early delivery with close intrapartum monitoring is necessary.In pregnancies with no specific medical indication for delivery in the 41st week a policy of delaying induction until the 42nd week reduces the rate of induction because about 80% of women have spontaneous onset of labor.

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The rate of cesarean section for failure to progress or fetal distress is about 15% in those with spontaneous onset of labor and 35% in those requiring induction of labor.

The chance of spontaneous onset of labor is higher in parous than nulliparous women and is inversely related to maternal body mass index and cervical length.

The chance of vaginal delivery is higher in those with spontaneous than induced onset of labor, in parous than nulliparous women, and in White than Black women and is inversely related to maternal body mass index and cervical length.

Outcome calculatorWhen you obtain the FMF Certificate of competence in cervical assessment you will have access to this calculator which will allow you to predict the chance that a patient at 40-41 weeks will go into spontaneous labor in the subsequent 10 days and whether she will end up having a cesarean section.

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