Can a Routine Scan Help Prevent Premature Birth? Cervix Screening and Progesterone Explained

A 2026 review of the research on checking every pregnant woman's cervix length (Zethelius and colleagues, Acta Obstetricia et Gynecologica Scandinavica)

Some hospitals offer every pregnant woman an ultrasound that measures the length of the cervix, the "neck" of the womb, and give a hormone called progesterone to those whose cervix is short, in the hope of preventing premature birth. A 2026 review of the research found that we still do not know for sure whether doing this for everyone lowers the number of babies born too early. The evidence is promising in places but not strong enough yet to be certain.

Why premature birth matters so much

A baby is "premature" or "preterm" if born before 37 weeks of pregnancy, instead of the usual 40. In 2020, about 13.4 million babies worldwide were born early, which is roughly one in every ten births [1]. Most early births start on their own, without a doctor deciding to deliver the baby, and about half of the women who have one had no known warning signs beforehand [2]. This is one reason doctors are so interested in finding a test that could warn about early birth in any pregnancy.

Being born early can make life harder for a baby, especially one born very early. These babies often need care in a neonatal intensive care unit (NICU), and some face longer-term health or learning challenges [3]. Even a small delay in birth, giving a baby a few more days or weeks to grow, can make a difference. That is why research on preventing early birth is taken so seriously.

How doctors came to look at the cervix

The cervix is the lower end of the womb. It normally stays long and firmly closed until labour begins. In the 1990s, a large study in the United States showed that women with a shorter cervix on an ultrasound in the middle of pregnancy were more likely to go into labour early [4]. Doctors then asked whether a short cervix could be treated. Several analyses that combined results from many studies found that progesterone, a natural hormone, lowered the chance of early birth for women with a short cervix [5][6].

For many years, though, most of those studies involved women who were already known to be at higher risk. A different question remained: should every pregnant woman be offered the scan, even if she has no risk factors? This is called "universal screening." A short cervix is uncommon in low-risk pregnancies, and doctors have not agreed on exactly how short is "short" [7]. In 2022 an international ultrasound society recommended universal screening [8], while many national groups kept recommending the scan mainly for women at higher risk of early birth. A Swedish study using a computer model suggested universal screening could be good value for money [9], but computer models rely on assumptions about how well treatment works, and that is exactly what is still in question.

What the researchers did

A team of doctors, health-technology experts and librarians in Sweden searched the medical literature for studies that compared two groups of women with a single baby on the way and no signs of early labour: women offered a cervix scan, with progesterone if the cervix was short, and women who were not offered the scan. They registered their plan in advance, which helps keep a review honest, and searched until September 2025 [10].

They found only six studies that met their rules. Two were randomised trials, where women are assigned by chance to one approach or the other, which is the fairest way to compare. One was done at a single hospital in India with 300 women [11]. The other, called the Preterm Birth Screening Study, was done at two centres in Italy with 1,334 women [10]. The other four were cohort studies, which look at what happened to large groups of women before and after a hospital started a screening programme. They came from France, Spain, the United States and Greece, and together covered more than 425,000 women [10], including 41 French maternity centres [12] and one large U.S. centre [13].

What they found

In the two trials, women who were screened did not have noticeably fewer early births. In the Italian trial, about 7.5 out of every 100 screened women had a baby before 37 weeks, compared with about 8.7 out of every 100 who were not screened. That difference was small enough that it could have happened by chance. The babies' health was also similar in both groups: for example, in the two trials combined, serious problems in newborns happened in about 5 out of 100 babies either way [10].

In the large before-and-after studies, early birth was lower after screening programmes began. For example, babies born before 37 weeks fell from about 6.0 out of every 100 to about 5.6 out of every 100. The researchers worked out that roughly 263 women would need to be screened for one early birth to be avoided [10]. None of these studies reported how the babies themselves did, so we cannot tell from them whether the newborns were healthier.

Why the answer is still "we're not sure"

So why do the two kinds of study disagree? The review explains that both have weaknesses. The trials were too small. To see a real but modest benefit, a trial like the Italian one would have needed about 16,228 women, not the 1,334 who took part [10]. When a trial is too small, "no difference found" does not mean "no difference exists."

The before-and-after studies are bigger, but they can be fooled. Things other than screening might have changed between the two time periods, such as how hospitals cared for women. In France, for instance, more than a quarter of women in the "before" period had the scan anyway [10]. Also, in many places women with a short cervix were offered other treatments, such as a soft ring placed around the cervix called a pessary, or a stitch, so it is hard to say how much was due to progesterone itself. You can read more about pessaries in our article on cervical pessary for short cervix.

An earlier review that mixed trials and cohort studies together had reported a slight reduction in early birth [14], but the Swedish team argues that mixing the two types lets the less reliable studies steer the result [10]. The experts rated the overall certainty as "very low," which is their way of saying the true answer could be quite different from what the studies suggest. This is not unusual in medicine. Treatments that seem sensible and safe sometimes fail when tested in big trials; one example was a hormone injection called 17-OHPC, tested to prevent repeat early births in the PROLONG trial, a large international study that did not confirm earlier hopes [15].

What this means for you and your baby

If you are pregnant, your care team will decide what scans and treatments are right for you, and this review does not change that. It does mean a few things.

If you have already had a premature baby, or have another known risk, your doctor may still recommend treatment such as progesterone; this review was about women without those risks, and evidence for women at high risk is stronger [16]. If you are low risk and your hospital offers a cervix scan, this review did not examine possible harms in detail, and it found no proof yet that the scan prevents early birth. If your hospital does not offer it, that does not mean you are being given worse care.

If you are told you have a short cervix, it can feel frightening. It may help to know that this finding raises the chance of an early birth but does not mean it will happen, and that progesterone may lower the risk somewhat even though the size of that benefit is still being studied. Our article on antenatal steroid timing explains one of the other steps doctors take if early birth looks likely. It is always fine to ask your doctor or midwife what the scan result means for you.

What researchers are working on next

The authors call for bigger, better trials before anyone decides whether every pregnant woman should be offered this scan. One idea is to introduce screening region by region over time within normal care, so that the effect can be fairly compared while everyone still receives usual care. Another is to combine detailed data from many earlier studies. Researchers also want to learn when in pregnancy the scan works best and what length of cervix should trigger treatment. Importantly, future studies should follow the babies' health, not just the week of birth [10].

A note on how to read the numbers

The percentages in this article can sound tiny, and in a way they are: the difference between 6.0 and 5.6 early births per 100 women is less than half a baby per hundred pregnancies. But because so many women are pregnant each year, if the cohort results turned out to be real, even small shifts could add up to many families spared a NICU stay, which is why researchers care about getting the answer right. It is also why the cost of screening everyone, in scans, appointments and worry for women who turn out to be fine, has to be weighed carefully against the benefit.

The bottom line

Whether screening every pregnancy for a short cervix prevents premature birth is a question science has not yet answered. The studies done so far were too small or too open to bias to give a firm yes or no. For families, the most useful message is to talk with your care team about your own situation, and to know that researchers are working to find out which approach really gives babies the best start.

References

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  2. Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes of preterm birth. Lancet. 2008;371(9606):75-84. doi:10.1016/S0140-6736(08)60074-460074-4) ↩
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