What the problem is

Tommy's describes it simply: "Sometimes the neck of the womb (the cervix) shortens and opens too soon in pregnancy. This can lead to your baby being born prematurely (before 37 weeks)."

The names are a mess and some of them are unkind. Tommy's lists weak cervix, weakened cervix, cervical incompetence and cervical insufficiency, and notes that "some women and birthing people do not like these terms and prefer to call it 'preterm cervical shortening'." Nothing in any of those words implies you failed at something.

The hardest feature is that it is usually silent. Tommy's: "Your cervix can shorten and open without any pain or other symptoms. It is usually only diagnosed when a person has had 1 or more late miscarriages or premature babies, and other causes have been ruled out."

Some people do get symptoms. Tommy's lists smelly and/or yellow or green vaginal discharge, bleeding, or a feeling of pressure low in the tummy or vagina, and says to tell your maternity team about those or anything else worrying you.

Why a cervix behaves this way

The RCOG's cervical stitch leaflet lists the histories that should trigger an early referral to a specialist:

a previous miscarriage after 16 weeks of pregnancy
a previous birth before 34 weeks
waters breaking before 34 weeks in a previous pregnancy
certain treatments to the cervix, such as LLETZ or a cone biopsy for abnormal cervical screening results
scarring of the endometrium, or an unusually shaped uterus
a previous caesarean birth at full dilatation
having needed a cervical stitch in a previous pregnancy

Tommy's adds that some people are simply born with a cervix prone to this, including where there is a differently shaped uterus or a collagen condition such as an Ehlers-Danlos syndrome, and that the cervix can also be injured during a previous labour.

The scan, and what 25 mm means

The measurement is done by transvaginal ultrasound. The NHS says the scan is usually offered between 16 and 24 weeks, and that "having a cervix that is shorter than 25mm has been linked to a higher risk of early labour and birth." Tommy's gives the same threshold.

NICE turns that into a decision rule. Recommendation 1.2.1 says to offer a choice of prophylactic vaginal progesterone or prophylactic cervical cerclage to people who have both a history of spontaneous preterm birth up to 34+0 weeks or loss from 16+0 weeks, and a cervical length of 25 mm or less on a scan between 16+0 and 24+0 weeks — with the risks and benefits of both discussed and the decision shared.

Where only one of those two applies, NICE says to consider vaginal progesterone. NICE separately says to consider prophylactic cerclage where the scan shows 25 mm or less and there has been either preterm prelabour rupture of membranes in a previous pregnancy or a history of cervical trauma.

On timing, NICE says vaginal progesterone should be started between 16+0 and 24+0 weeks and continued until at least 34 weeks. Tommy's notes there is no evidence that one of the two treatments is better than the other, which is precisely why NICE frames it as a choice.

What a cervical stitch actually is

The RCOG: "A cervical stitch is a procedure in which a stitch is placed around your cervix (the neck of your womb) to try to keep it closed. It is usually done between 11 and 24 weeks of pregnancy, as a planned procedure." It is also called a cervical suture or cervical cerclage.

Most are put in through the vagina. The RCOG describes the procedure: it takes place in an operating theatre, under either a spinal anaesthetic (awake, numb from the waist down) or a general anaesthetic; a speculum is inserted, the stitch is placed around the cervix and tightened; the operation takes less than an hour, and most people go home the same day or early the next.

Removal is usually at 36 to 37 weeks, unless labour starts sooner, or at the time of a planned caesarean. The RCOG notes that taking it out is simpler than putting it in and does not normally need an anaesthetic.

There are two variants. A transabdominal stitch may be recommended if a vaginal stitch has not worked before or is not technically possible; the RCOG says it is ideally done before pregnancy, is not removed, and means the baby is born by caesarean. An emergency stitch may be offered after the cervix has already started to open; the RCOG is candid that "this type of stitch has higher risks and is less likely to work than other stitches." NICE says emergency cerclage can be considered between 16+0 and 27+6 weeks with a dilated cervix and exposed, unruptured membranes, after discussion with a consultant obstetrician and consultant paediatrician.

When a stitch is not the answer

NICE says not to offer emergency cervical cerclage where there are signs of infection, active vaginal bleeding, or uterine contractions. The RCOG lists the same contraindications for planned stitches, adding waters that have already broken.

Multiple pregnancy is a separate case. The RCOG: "If you are pregnant with more than one baby, there is no evidence to show that a cervical stitch will prevent you going into labour early and your care will be individualised depending on your situation."

The RCOG also lists the risks honestly: bleeding, infection, injury to the bladder or the cervix, and waters breaking early. "Even if the stitch is put in successfully it may not always work and you might still experience a late miscarriage or preterm birth." It does not increase your chance of needing a caesarean.

Afterwards, and what to watch for

The RCOG says to expect some vaginal bleeding or brownish discharge for a day or two, to use a pad rather than a tampon, and to rest for the first couple of days. After that, "you can carry on as normal for the rest of your pregnancy... Resting in bed is not routinely recommended." Having a stitch does not affect the baby's growth and development, and sex is fine when you feel comfortable.

The RCOG says to contact your healthcare team without delay if you have:

contractions or cramping abdominal pain
continued or heavy vaginal bleeding
your waters breaking
smelly or green vaginal discharge

If you go into labour with a vaginal stitch in place it should be removed as soon as possible, so tell whoever assesses you that you have one — say it at the door, not twenty minutes in.

Getting the right care in the next pregnancy

The single most useful thing after a late loss or an early birth is an early referral. Tommy's says that if you have previously had a late miscarriage or given birth before 34 weeks, or are at risk of a weak cervix, "you should be referred to a specialist early in your pregnancy to plan your care", and notes that people with three or more preterm births or second trimester losses may be offered a stitch on history alone, usually between 11 and 14 weeks.

Detailed clinical practice in the UK follows the RCOG's Green-top Guideline No. 75 on cervical cerclage; ACOG's practice bulletin on cerclage for the management of cervical insufficiency covers US practice, which uses similar thresholds. If you are told there is nothing to be done, that is worth a second opinion.

Cervical Insufficiency and Cerclage