Cervical Length Screening and Preterm Labor

Introduction

Preterm labor, which occurs between 20 and 37 weeks of pregnancy, is a major global health and obstetric concern, impacting five to 18 percent of pregnancies worldwide. As the primary cause of neonatal mortality, preterm birth is also associated with significant morbidity in surviving infants. Early cervical assessment may help identify pathological changes before preterm birth symptoms occur, aiding in prevention.

What Is Preterm Labor?

Preterm labor happens when a baby is born before 37 weeks of pregnancy, and it is a serious issue that affects about 15 million babies worldwide every year. The birth order of a baby can also impact their health at birth and later in life. Several factors can cause preterm births, including infections, problems with the cervix, an overstretched uterus, hormonal issues, stress for both mother and baby, and immune system reactions, along with some causes that are still unknown.

What Is Preterm Parturition Syndrome?

Preterm parturition syndrome is a complex condition where various factors contribute to preterm labor, with preterm labor being the expected outcome. Spontaneous preterm birth (sPTB) occurs due to the activation of one or more pathological triggers that initiate early labor. These triggers can be categorized into anatomical, physiological, biochemical, hormonal, immune-related, and clinical factors that affect both the mother and the fetus.

What Is the Role of Cervical Length in Predicting Spontaneous Preterm Birth?

Cervical shortening is one of the early signs that can occur weeks before the onset of spontaneous preterm birth. Measuring cervical length (CL) via transvaginal ultrasound (TVUS) during the mid-trimester is a method for predicting sPTB. The risk of sPTB decreases as cervical length increases, with women who have both a history of preterm birth and a short cervix being at the highest risk.

What Is the Best Time for Cervical Length Screening During Pregnancy?

Most clinical guidelines recommend cervical length screening between 16 and 24 weeks of pregnancy for asymptomatic women with a history of preterm birth. Screening before 16 weeks is generally not recommended, as the accuracy of predicting PTB in the first and early second trimester is low, especially for women without symptoms or a history of PTB. Routine cervical length screening after 24 weeks is also not recommended. However, even beyond routine screening, early detection of a short cervix can still offer benefits for asymptomatic women, such as transfer to a specialized care facility, admission to a high-risk unit, and the use of corticosteroids and magnesium sulfate to support fetal development. In addition, measuring cervical length is primarily relevant for pregnant women, as such measurements in non-pregnant women do not have predictive significance for spontaneous preterm birth.

How Can Cervical Length Predict Preterm Birth?

The cervix, located at the lower end of the uterus, plays a vital role in indicating when the body is getting ready for childbirth. One way doctors predict preterm birth is by measuring cervical length. Now, transvaginal ultrasound is considered a much more accurate method.

There are a few ways to measure cervical length using ultrasound, including transabdominal (TAU), transperineal (TPU, also called translabial), and TVU. Shortening or funneling of the cervix, which can be detected through ultrasound, has been linked to preterm birth. TVU is especially useful for women at high risk, though there is some debate about whether it should be routinely used in women at low risk of preterm delivery.

Cervical insufficiency is a condition where the cervix can not hold a pregnancy without contractions. In general, the shorter the cervix, the higher the risk of preterm birth. But most women (about 75 percent) with short cervix still go on to have full-term pregnancies.

A cervical length of less than 25 millimeters is considered short and may suggest a higher risk of preterm birth, especially before 28 weeks of pregnancy. Women with short cervix and contractions are at an even higher risk of delivering early compared to those with short cervix but no contractions.

How Does a Short Cervical Length Develop?

Three main factors contribute to the development of a short cervical length (CL):

• Cervical Insufficiency: This is when the cervix is weak, often due to past trauma, surgery, or, more rarely, a congenital condition or connective tissue disorder. Interestingly, even women at high risk usually have a regular cervical length during the first trimester, as the growing gestational sac does not exert enough pressure to affect the cervix until later in pregnancy. This is why screening before 14 weeks could be more effective.

• Infection or Inflammation: Another possible cause is an infection or inflammation. There is a strong link between a short cervical length detected through ultrasound and the presence of infection.

• Contractions: Research has shown that most women with short cervical length (less than 25 mm) before 24 weeks experience contractions. It is unclear whether the contractions cause the cervix to shorten or if the short cervix triggers contractions. These factors may also work together.

These mechanisms, along with others, likely combine in different ways for different women to contribute to developing a short cervical length. Additionally, the risk of developing a short CL is higher in women with a history of preterm birth, previous cervical surgery (such as a cone biopsy or large loop excision of the transformation zone procedure), or those who have undergone procedures like dilation and curettage. Women carrying multiples are also at increased risk.

What Are the Interventions to Prevent Preterm Labor?

• Bed Rest and Hydration: Commonly suggested but lacks evidence for delaying delivery.

• Tocolytic Medications: Can delay labor for 24 to 48 hours but not beyond, with limited long-term effectiveness.

• Cervical Cerclage (Procedure to Place Stitches Around the Cervix): Prophylactic is done for high-risk women or those with low cervical resistance. Ultrasound indicated cerclage reduces PTB in women with a prior PTB and short cervix. Rarely, it is also used when the cervix is dilated but without signs of labor or infection. In low-risk patients, it is generally not effective.

• Vaginal Progesterone: Reduces PTB rates and respiratory distress in women with short cervix, though less effective in twin pregnancies.

Conclusion

A shortened cervical length is considered a significant indicator of preterm labor. Because of its reliability and accuracy, ultrasound is usually the preferred method for measuring the cervix. In addition, transvaginal ultrasound is the most precise technique available.