“Do They Check Your Cervix at the 20-Week Scan?” Short Cervix, Funnelling and Missed Risk Factors
In this guide
If you are wondering whether your cervix was checked at your 20-week scan, or you lost a baby or had one very early and later learned your cervix was short, this guide is for you. It explains how the cervix is measured, what the numbers mean, which risk factors should lead to closer watching, what should happen after a short result, and how to tell from your records whether something was missed.
Quick answer · updated November 2026
Often, but not always. RANZCOG supports measuring the cervix at the 20-week (morphology) scan, usually through the tummy, with an internal scan if it measures under 35 mm or can’t be seen clearly. A cervix of 25 mm or less on the internal scan is short and should lead to a plan. What your number means.
Your cervix through pregnancy, stage by stage
- 1BookingYour history is taken: past early births or losses, LLETZ, IVF.
- 2The referralYour risk factors should travel with your scan request.
- 3The 20-week scanThe cervix is measured, often through the tummy first.
- 4The internal scanIf it’s under 35 mm or unclear, a vaginal scan should follow.
- 5The planA short result should lead to progesterone, more scans or a stitch.
At any stage: you can ask, “Was my cervix measured, how, and what was the number?” It is your information.
I remember the gel being cold, and the sonographer turning the screen so I could see his profile. “Look at that nose,” she said. “Everything looks lovely.” I asked if there was anything else they checked and she said, “Just the usual,” and printed off three pictures for the fridge.
It was my first pregnancy. Years earlier I’d had a LLETZ after an abnormal smear, and I’d ticked the box for it on the booking form. I didn’t think about it again. Nobody mentioned it.
At twenty-three weeks I noticed more discharge, sort of watery, and a heaviness. They did an internal scan in the assessment unit, and the doctor’s face changed. “Your cervix is very short,” she said. “Nine millimetres. Has anyone talked to you about this before?” No one had.
They gave me steroid injections for his lungs. He was born at twenty-five weeks and went to the neonatal unit, and for the next three months my life was the sound of monitors.
I kept going back to that dark room with the cold gel. Just the usual.
A composite story, drawn from the experiences of many families I have worked with, combined and changed so that no one can be identified. It does not describe any individual client.
If you are reading this because you’ve just been told your cervix is short, I hope it helps you understand the plan and ask the right questions. If you are reading it afterwards, wondering whether your cervix should have been checked or followed up, I am sorry, and I hope it helps you find out.
This article is part of a series on failed cerclage and second-trimester loss. It explains how the cervix is measured at the 20-week scan, what the measurements mean, which risk factors should lead to closer watching, what should happen after a short result, and how the law looks at it when that doesn’t happen.
Why I can write this
I have been a medical negligence lawyer since 2013 and have worked on many hundreds of claims, a great many of them about pregnancy and birth. My doctorate looked at patterns of failure in medical negligence. I am not medically qualified. The clinical content here comes mainly from the Royal Australian and New Zealand College of Obstetricians and Gynaecologists’ statement on measuring cervical length, which I have read in full, and every source is listed at the end.
Start with your question
- Do they check your cervix at the 20-week scan?
- Is it measured through the tummy or internally?
- What is a normal cervix length at 20 weeks?
- What is considered a short cervix?
- What does funnelling mean?
- Who should have their cervix watched more closely?
- What should happen after a short cervix result?
Do they check your cervix at the 20-week scan?
In Australia, the honest answer is: often, but it depends where you are and how the scan is done. RANZCOG’s statement says that in some places the length of the cervix is assessed routinely at the mid-pregnancy scan at around 20 weeks, and in others only for women with risk factors or symptoms [1]. It notes that every pregnant woman in Australia and New Zealand is offered a mid-trimester ultrasound, and that cervical length can be assessed at that scan [1].
RANZCOG currently supports measuring the cervix through the tummy at the mid-trimester scan for women at low risk with one baby, and adding an internal scan for those whose cervix measures under 35 mm or can’t be clearly seen [1]. It describes a Western Australian statewide program built on exactly that approach, which lowered the state’s preterm birth rate by 7.6% in its first full year, and says the program has since been accepted for implementation by every state and territory [1].
So if your report doesn’t mention your cervix at all, or says it couldn’t be seen and nothing followed, that is worth asking about.
How the cervix is measured
Definition
Cervical length is the length of the cervical canal, measured on ultrasound from the inner opening (the internal os) to the outer opening (the external os) [1].
In plain terms: how long the neck of the womb is. In the middle of pregnancy, the shorter it is, the higher the chance of an early birth.
There are two main ways to measure it. A transabdominal scan is done through the tummy, as part of the usual morphology scan. A transvaginal (internal) scan uses a slim probe placed in the vagina. RANZCOG says the internal scan is the most accurate, and sets out a standard technique: an empty bladder, the probe placed so it doesn’t press on the cervix, and three measurements over about five minutes, with the shortest reported [1].
The tummy scan has real limits. RANZCOG notes that with a full bladder it overestimates length by about 6 mm compared with the internal scan, and with an empty bladder the cervix can’t be adequately seen in 18% of women [1]. That is why the 35 mm trigger exists: a tummy measurement that looks borderline, or a cervix that can’t be seen properly, needs the internal scan to know the real length.
If the idea of an internal scan feels confronting, you aren’t alone. RANZCOG reports that it is safe in pregnancy, and that most women find it causes mild or no discomfort [1]. You can ask for a support person to be with you.
What the numbers mean
RANZCOG cites research measuring the cervix at 16 to 22 weeks, which found an average length of 38.5 mm, with shorter lengths carrying a higher risk of early birth [1]. In a large screening program in low-risk women, only 1.3% had a cervix under 25 mm [1].
Cervical length at mid-pregnancy
38.5 mm was the average length at 16 to 22 weeks in the research RANZCOG cites [1].
Under 35 mm on a tummy scan, or not clearly seen, should lead to an internal scan.
25 mm or less on an internal scan is short: vaginal progesterone was started nightly in the program RANZCOG describes.
Under 10 mm: a stitch could be considered.
In plain terms: the number tells you what should happen next, so ask for it.
These thresholds come from the statewide program RANZCOG describes [1]. Your own plan may differ with your history: for example, if you’ve had an earlier preterm birth or late loss, a stitch may be considered at a longer length. RANZCOG also notes that the Therapeutic Goods Administration has approved vaginal progesterone for preventing preterm birth in women with a mid-trimester cervix of 25 mm or less, or a previous spontaneous preterm birth [1].
Funnelling and other words on the report
Definition
Funnelling is effacement of the internal part of the cervix [1]. Amniotic debris, sometimes called sludge, is material seen floating in the fluid near the cervix [1].
In plain terms: funnelling means the inside top of the cervix has started to open like a funnel. Sludge can be linked to inflammation or infection.
RANZCOG notes that funnelling is associated with early birth, though it may add little beyond an accurate length measurement, and that sludge in women with a short cervix is an independent risk factor, linked with inflammation and infection of the membranes [1]. If either word appears on your report, it is worth asking what was done about it. Other phrases worth noticing are “not well visualised”, “suboptimal views”, and “recommend transvaginal assessment”: each one should have a follow-up.
Who should have their cervix watched more closely
A single measurement at 20 weeks is a screening test. Some women need more than that, from earlier in pregnancy. RANZCOG lists risk factors for spontaneous early birth, and also notes that two-thirds of women who give birth early have no recognisable risk factor at all [1].
Tell your midwife or doctor, and check it’s on your scan referral, if you have had
- An earlier baby born before 37 weeks
- A miscarriage or loss in the second trimester
- LLETZ, a cone biopsy or other treatment to the cervix
- A cervical stitch in an earlier pregnancy
- IVF or other assisted conception
- A difference in the shape of your uterus, or fibroids
- Twins or more in this pregnancy
- A caesarean late in labour, at full dilatation
In plain terms: if any of these apply, your cervix may need watching more than once, and earlier.
Sources: RANZCOG C-Obs 27 [1]; full-dilatation caesarean from the BJOG review of cerclage [2].
LLETZ and cone biopsy
This is the risk factor I see missed most often, because it happened years earlier, for a different reason, and was treated by a different doctor. RANZCOG says treatment that removes tissue from the cervix is associated with higher rates of preterm birth at all gestations, that the risk rises with the amount of tissue removed and with repeat procedures, and that a mid-trimester cervical length under 25 or 30 mm in these women gives a much higher chance of early birth [1]. Measuring the cervix, it says, can help sort out who is at higher risk [1].
A previous early birth or late loss
RANZCOG says women with a previous preterm birth may benefit from vaginal progesterone or a stitch, and that there is evidence for watching the cervix with scans and placing a stitch only if it becomes short [1]. If you had a late loss before, this pregnancy should start with a plan, not wait for the 20-week scan. The series article “Pregnant Again After a Second-Trimester Loss” covers that plan.
Twins
Cervical length still predicts early birth in twins, but RANZCOG notes that the evidence for treatment of a short cervix in twin pregnancies is conflicting [1]. Knowing the length still helps teams plan where and how a very early birth would happen.
Questions to ask
- “Was my cervix measured at the 20-week scan? Through my tummy or internally?”
- “What was the number? Was it clearly seen?”
- “Does my scan referral mention my LLETZ, my earlier loss or my IVF?”
- “Should I have more than one cervical scan this pregnancy?”
- “Should I be seen in a preterm birth or high-risk clinic?”
What should happen after a short cervix result
A short cervix is not a diagnosis that labour is coming. RANZCOG notes that most women with a short cervix in mid-pregnancy still give birth at term, and describes a “latent phase” of relative stability before more rapid change, which gives time to act [1]. That window is exactly why a short result matters: there is usually time to do something, if someone does it.
Tummy scan
Under 35 mm, or not clearly seen?
Internal scan
Measured the standard way, shortest of three.
25 mm or less
Vaginal progesterone, closer watching, and your history reviewed.
Very short or shortening
A stitch may be discussed, especially after a past early birth or loss.
Based on the program and evidence described in RANZCOG C-Obs 27 [1]. RANZCOG also notes that a stitch may help women whose cervix keeps shortening despite progesterone, and that progesterone is generally preferred for an otherwise low-risk woman with a short cervix because it carries less surgical risk.
RANZCOG also mentions practical barriers that matter in real life: progesterone costs money and needs to be stored below 25 degrees, and access to internal scans may be limited in rural and remote areas [1]. If any of these got in the way for you, it is part of your story too. The different kinds of stitch, and how well each works, are explained in the hub article “What Happens If a Cerclage Fails?”, and an urgent stitch in “Emergency Cerclage at 20 Weeks”.
If you were told your cervix was short and later developed pressure, cramps or discharge, “How Do You Know If Your Cerclage Is Failing?” explains the warning signs and what should happen when you report them.
When my son was nine months old and finally sleeping more than two hours, I asked for my records. I told myself it was for his baby book.
The booking form was the first page. There was my tick: Previous cervical surgery: LLETZ. Then the referral for the morphology scan, which had a box for “relevant history”. It was empty.
The scan report said everything about him was normal. Underneath, one line: “Cervix not clearly visualised transabdominally.” No recommendation, no internal scan, nothing in my notes afterwards.
I’d spent nine months thinking my body had done this out of nowhere. Reading that line, I felt angry first, and then something closer to steadiness. My history had been written down. It just hadn’t travelled to the room where it mattered.
When I talk to my friends who are pregnant now, I tell them one thing. Ask what your cervix measured. Ask how. And make sure the box isn’t empty.
A composite story, drawn from the experiences of many families I have worked with, combined and changed so that no one can be identified. It does not describe any individual client.
What to look for in the records
| Record | Why it matters |
|---|---|
| Booking history | Shows whether LLETZ, cone biopsy, earlier preterm births or losses, IVF or uterine differences were recorded. |
| Morphology scan referral | Shows whether your risk factors were passed on, and whether cervical length was requested. |
| Morphology scan report | Shows whether the cervix was measured, how, the number, how clearly it was seen, and any recommendation. |
| Any internal scan report | Shows the true length, funnelling or sludge, and the technique used. |
| Antenatal notes after the scan | Shows whether the result was read, discussed with you and acted on. |
| Clinic letters and referrals | Shows whether you were referred to a preterm birth or high-risk clinic. |
| Medication chart or prescription | Shows whether progesterone was offered, prescribed and started. |
| Later scans | Shows whether the cervix was measured again, and how it changed. |
| Assessment unit and birth notes | Shows what was found when you presented with symptoms, and when. |
My guide to getting and reading your birth records explains how to ask for them, and the Information and Privacy Commission explains your right to your own health information in NSW [3]. Ask for the scan reports from the imaging practice too, if your scan was done privately.
How the law looks at it
A short cervix that wasn’t found is not automatically negligence. The law asks two main questions.
Was the care below standard?
Not negligent if widely accepted in Australia by peer professional opinion as competent practice [4].
In plain terms: should your history have been passed on, your cervix measured internally, or a short result acted on?
Did it change the outcome?
Better care must probably have changed what happened. A lost chance alone is not enough [5].
In plain terms: with progesterone or a stitch, would your baby probably have been born later?
Were you properly informed?
You are entitled to be told about material risks and reasonable options [6].
In plain terms: were you told your cervix was short, and what could be done?
RANZCOG’s statement is evidence of what competent practice looks like, not the law itself, and its screening recommendation is described as consensus-based [1]. Practice also varies between services. So an expert reviewing your care will ask what a careful service would have done for a woman with your history, at your hospital, at that time. Section 5O of the Civil Liability Act 2002 (NSW) sets that test.
Causation is often the harder question. Progesterone and a stitch reduce the risk of early birth; they don’t remove it. Under Tabet v Gett [2010] HCA 12, it isn’t enough that treatment would have given a better chance; it has to be probable that it would have changed the outcome. Independent obstetric opinion, and often the pathology results, decide that. Under Rogers v Whitaker [1992] HCA 58, you are entitled to be told about a short cervix and your options.
Who can claim depends on what happened: the mother for her own injury; the family if a baby was born alive and died; or the child, if a baby survived with lasting injury from being born early. If your child was born early, their time limits don’t always wait until they turn 18, so please ask early. The series article “Can I Sue the Hospital for My Miscarriage or Stillbirth?”, my page on pregnancy loss and stillbirth claims and my guide to time limits in NSW explain more.
In my practice
The cervical length files I see rarely turn on a sonographer measuring badly. They turn on handovers. A history written on a booking form that never reaches the scan referral. A report that says “not clearly visualised” with no one responsible for booking the internal scan. A short result filed in the notes and not read at the next appointment, because the appointment was about something else. A prescription for progesterone that was never filled because nobody explained why it mattered or what it cost. Each step is someone else’s job, and each gap is small. My research looked at exactly these chains, because they are where harm usually lives, and also where systems can most easily change. Not every chain ends in negligence; sometimes the cervix changed between scans faster than any schedule would have caught. But it is always worth looking.
Both can be true
The sonographer who showed you your baby’s profile may have done her job well that day, and the system around her may still have failed you. You can be grateful your baby was cared for in the neonatal unit and still want to know why your cervix wasn’t measured properly. Asking is not ingratitude. It is how the empty box on the next woman’s referral gets filled in.
The bigger picture
Preterm birth is the leading cause of death and illness in newborns, and RANZCOG notes that in Australia it accounts for almost 70% of perinatal deaths [1]. Cervical length screening is part of a national shift in how preterm birth is prevented: the statewide program RANZCOG describes has been accepted for implementation in every state and territory [1], and the Australian Preterm Birth Prevention Alliance, a committee of the Perinatal Society of Australia and New Zealand, exists to safely lower the rate of preterm birth in Australia [7]. RANZCOG also flags real gaps: no national data on cost-effectiveness, uncertainty about who pays for the extra internal scans, and limited access in rural and remote areas [1]. Its statement is due for review this month. In NSW, the Select Committee on Birth Trauma reported on 29 May 2024, with 43 recommendations [8].
Support
These services are free.
- Pregnancy, Birth and Baby, a free call with a maternal child health nurse: 1800 882 436 [9]
- Miracle Babies Foundation NurtureLine, 24-hour support for families with a threatened pregnancy or a baby in NICU: 1300 622 243 [10]
- Red Nose Grief and Loss, 24/7 support after pregnancy loss, stillbirth or the death of a baby: 1300 308 307 [11]
- PANDA, the National Perinatal Mental Health Helpline: 1300 726 306 [12]
- Lifeline, if you are in crisis: 13 11 14
My guide to finding support after birth trauma in Australia has more.
The mother in the stories has the three scan pictures on her fridge still, next to one of her son, now walking. She has stopped looking at them as the day something was missed, and started looking at them as the day she learned to ask.
With care,
Rosemary
Frequently asked questions
Do they check your cervix at the 20-week scan?
Often, but not everywhere. RANZCOG supports measuring it at the mid-pregnancy scan, usually through the tummy, with an internal scan if it is under 35 mm or can’t be seen clearly.
What is a normal cervix length at 20 weeks?
Research cited by RANZCOG found an average of about 38.5 mm at 16 to 22 weeks. Shorter lengths carry a higher chance of early birth.
What is considered a short cervix at 20 weeks?
Generally 25 mm or less on an internal scan. Under 35 mm on a tummy scan should lead to an internal scan to check.
What happens if you have a short cervix at 20 weeks?
Usually vaginal progesterone and closer watching. A stitch may be considered if it is very short, keeps shortening, or you have had an earlier early birth or loss.
Does LLETZ cause a short cervix?
Treatment that removes cervical tissue is linked with a higher rate of preterm birth, rising with the amount removed. RANZCOG says measuring the cervix helps sort out who is at higher risk.
Can I ask for an internal scan of my cervix?
Yes. If you have a risk factor, or your cervix couldn’t be seen clearly, ask your midwife or doctor whether you should have one, and why not if they say no.
Questions about your cervix scan?
When you’re ready, and not before, I can go through what happened and your records with you, and tell you honestly whether there are questions worth asking.
When the Stitch Doesn’t Hold: the series
- “What Happens If a Cerclage Fails?” Failed Cerclage and Losing a Baby in the Second Trimester
- “Can I Sue the Hospital for My Miscarriage or Stillbirth?” Time Limits and Compensation After a Second-Trimester Loss
- “Do They Check Your Cervix at the 20-Week Scan?” Short Cervix, Funnelling and Missed Risk Factors (you are here)
- “How Do You Know If Your Cerclage Is Failing?” Signs of a Failing Stitch or an Incompetent Cervix, and When They’re Brushed Off
- “Emergency Cerclage at 20 Weeks”: Rescue Stitches, Bulging Membranes, Risks and Success Rates
- “My Water Broke at 20 Weeks”: PPROM, Infection After Cerclage and What Happens Next
- “Baby Born at 22 Weeks”: Survival Rates, NICU and Comfort Care
- “Is 20 Weeks a Miscarriage or a Stillbirth?” Giving Birth, Funerals, Certificates, Leave and Payments
- “Why Am I So Angry After My Miscarriage?” Anger, Blame and “Is It My Fault?” After a Second-Trimester Loss
- “Transabdominal Cerclage in Australia”: Before Pregnancy, Success Rates and When It Fails
- “Pregnant Again After a Second-Trimester Loss”: How Long to Wait, a Preventative Stitch and Progesterone
More: Pregnancy loss and stillbirth claims · Birth trauma and obstetric negligence · Asking for a birth debrief · Complaining to a hospital · The birth injury map
References
- RANZCOG, Measurement of cervical length for prediction of preterm birth (C-Obs 27), current November 2021, review due November 2026.
- Story L, Shennan A, Cervical cerclage: an evolving evidence base, BJOG 2024;131(12):1579–1586.
- Information and Privacy Commission NSW, Fact sheet: Accessing your health information in NSW.
- Civil Liability Act 2002 (NSW), s 5O.
- Tabet v Gett [2010] HCA 12.
- Rogers v Whitaker [1992] HCA 58.
- Perinatal Society of Australia and New Zealand, Australian Preterm Birth Prevention Alliance.
- NSW Legislative Council Select Committee on Birth Trauma, Birth trauma, Report 1, May 2024.
- Pregnancy, Birth and Baby, Freecall numbers.
- Miracle Babies Foundation, NurtureLine.
- Red Nose, Grief and Loss Support Services.
- PANDA, National Perinatal Mental Health Helpline.
The stories in this article are composites, drawn from the experiences of many families I have worked with, combined and changed so that no one can be identified. They do not describe any individual client.
About the author
Dr Rosemary Listing, PhD (Law), specialises in medical negligence, has been in practice since 2013, and is Of Counsel at Peter Evans & Associates in Newcastle, NSW. Her doctorate looked at patterns of failure in medical negligence, and she teaches health law at the University of Newcastle. She is not medically qualified; clinical information in this article is checked against the sources listed.
Last updated: November 2026 · First published: November 2026 · Next review: May 2027
Cite this article: Rosemary Listing, ‘“Do They Check Your Cervix at the 20-Week Scan?” Short Cervix, Funnelling and Missed Risk Factors’ (Reframe Legal, November 2026) <https://reframelegal.com/do-they-check-your-cervix-at-the-20-week-scan/>.
This article is general information, not legal or medical advice. Every situation is different; please get advice about your own circumstances. If you have symptoms now, contact your hospital. If you or your baby need urgent help, call 000. If you are in crisis, call Lifeline on 13 11 14.
Legal services are provided by Peter Evans & Associates.
