The UK national guidance from NICE (National Institute for Health and Care Excellence) on how to best manage twin and triplet pregnancies had some small but important changes in 2024. One of the main changes was to recommend cervical length screening for all women and pregnant people with a twin or triplet pregnancy.
Why do cervical length screening?
Women with a twin or triplet pregnancy have a higher chance of starting labour early (spontaneous preterm birth) than women with a single baby.
It is not clear why preterm birth is more common in twin and triplet pregnancy but around 60% of twins will deliver before 37 weeks and 75% of triplets before 35 weeks.
In just under half of twin and almost all triplet births, at least one baby will need to go to the neonatal intensive care unit (NICU).
There are risk factors associated with preterm birth, including delay in achieving developmental milestones which may become clearer as your babies approach school age.
Does it matter what type of twins I have?
Cervical length screening is offered to all women carrying twins.
Women with dichorionic twins (each baby has its own placenta), remain at increased risk of spontaneous preterm birth and therefore could benefit from a cervical length screen.
Women with monochorionic twins (the babies share a placenta), have a chance of a number of specific problems related to their placenta which can lead to early birth. Unfortunately, even if these placental problems do not occur, there remains a risk of spontaneous preterm birth and therefore cervical screening may be helpful.
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How often will I have a cervical length screen?
A single cervical length screening test is recommended between 16 and 20 weeks of pregnancy.
As the cervical length measurement is a screening test, it is not recommended to be repeated unless you have new symptoms of concern, such as bleeding, watery discharge or abdominal pain or cramps.
What should I expect from a cervical length screen?
Having a cervical length assessment is normally well tolerated and many women will have had a similar test previously, such as in early pregnancy or during IVF.
It requires you to lie on your back with your legs apart and bent at the knee. A pen-like ultrasound probe, with a plastic sheath on with lubricating gel, is then placed in the vagina. This allows the clinician to see the cervix and measure the length of the cervical canal. The cervical length screen usually only takes a few minutes.
This procedure does not cause preterm birth or miscarriage and does not cause any harm or damage to you or your babies. There should be only minimal discomfort during the cervical length screen.
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What would happen if my cervix is short?
If your cervix is found to be short, usually 25mm or less, then your doctor or midwife may offer you treatment with progesterone.
Progesterone is normally given as a 200mg tablet or gel to put in the vagina once per day, usually at night. This treatment is usually continued until 34 weeks.
Other treatments are sometimes offered and your clinician will discuss these individually with you if they feel they are needed.
What is the scientific evidence for treatment based on a short cervix in twin and triplet pregnancy?
The scientific evidence for the benefit of treating a woman with a short cervix and twins is not strong, mainly due to the low number of studies that have been performed in twin pregnancies in this area. There is some evidence that women carrying twins and a short cervix who are treated with progesterone have a lower chance of preterm birth. There is no evidence for a benefit in triplet pregnancies, but it is reasonable to treat due to the high risk of preterm birth and the chance that there may be a benefit.
As the impact of a preterm birth in twins can be devastating, the use of cervical screening and treatment if a short cervix is found was advised.
Much of the evidence for what dose to use comes from pregnancies with a single baby and confidence that progesterone treatment does not seem to be harmful.
Will cervical length screening or treatment for a short cervix prevent me having a preterm birth?
No, having a cervical length screen will help to determine the chance of you having a preterm birth.
Treatment with progesterone for women with a short cervix may prevent a preterm birth but it is not a guarantee.
Unfortunately, some women with a normal cervical length screen and some with a short cervix who get treated will still go on to have a preterm birth. The test and treatment is to reduce the chance of preterm birth but will not remove it completely.
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