Slapped Cheek in Pregnancy: What the Biggest Parvovirus Wave on Record Taught Doctors

Four 2025 studies from Germany, a ten-country European group, Italy and the United Kingdom, explained for families

Europe's largest recorded outbreak of parvovirus B19 — the "slapped cheek" virus — swept through 2023 and 2024, and four studies published in 2025 now show what it meant for pregnancy. The reassuring finding is that the virus does not appear to have become more dangerous. The demanding one is that far more families faced it, and affected babies needed more treatment than before.

Parvovirus B19 causes fifth disease, a mild childhood rash. For almost everyone it is a few days of feeling unwell and nothing more. For a pregnant woman who has never had it, it can be different: the virus can reach the baby and attack the cells that make the baby's blood, causing severe anaemia and, in the worst cases, a dangerous build-up of fluid called hydrops. The four studies below trace that story from how many women were infected, through what happened to their babies before birth, to what the newborns looked like afterwards.

Why this virus matters before birth, and why the outbreak happened

Blood is made in the fetal liver during the middle months of pregnancy, and it is made fast. Parvovirus B19 happens to target exactly the cells that do this work. If a woman catches the virus and it crosses the placenta, the baby can become severely anaemic within a few weeks. Anaemia makes the baby's heart work much harder, and if it works hard enough for long enough, fluid starts to collect in the baby's abdomen, chest and skin — that is hydrops. Hydrops is the finding that matters most. A large review of 127 earlier studies, covering more than 1700 cases, found that babies who developed hydrops were far more likely to die than babies who were infected without it [1]. Infection in the first half of pregnancy is riskier than infection later on, a pattern first pinned down in a careful German study of over a thousand pregnancies [2].

Before the 1990s there was very little doctors could do. Now there is one treatment, and it is remarkable: a blood transfusion given to the baby before birth. A fine needle is passed through the mother's abdomen into a blood vessel in the umbilical cord or the baby's liver, and donor blood is given directly. To know when it is needed, doctors use a specialised ultrasound measurement — the speed of blood flowing through a vessel in the baby's brain, called the middle cerebral artery peak systolic velocity. Anaemic blood is thinner and flows faster, so a rising speed means falling blood counts, and it shows up before the baby becomes ill [3]. International guidelines built a whole pathway around this: if a mother's blood test confirms infection, she is scanned weekly, and if the velocity crosses a threshold, a transfusion is offered [4].

That system worked well for years, because the number of affected pregnancies was small. Then the pandemic changed the arithmetic. Parvovirus normally sweeps through communities in waves every three or four years. Masks, school closures and distancing during COVID-19 interrupted that cycle, so the wave that should have come around 2022 never arrived. By the time life returned to normal, an unusually large number of children and adults had never met the virus and had no immunity to it. Researchers in the Netherlands documented this gap in their national data [5], and Israeli researchers saw the same thing from 2023 [6]. When the virus came back, it had far more people to infect.

Four studies, four different questions

How much bigger was the wave? A group working across 13 specialist pregnancy centres covering all regions of Germany went back through ten years of records. Before the pandemic, those centres between them saw an average of about 57 infected pregnancies a year. During the pandemic, about 20. Afterwards, the average jumped to roughly 385. In the first four months of 2024 alone they recorded more cases than in the entire period from 2014 to 2022 combined, and more transfusions than in all those years put together. Projected across a full year, that is about thirteen times the pre-pandemic rate [7]. Centres in Denmark [8], northern Italy [9] and across northwestern Europe [10] all reported something similar.

Was the virus more dangerous? This is the question most families ask, and the answer appears to be no. In the German data, the share of infected pregnancies that developed hydrops, needed a transfusion, or ended in the baby's death stayed essentially the same before, during and after the pandemic — around one in ten developed hydrops and roughly one in eight needed a transfusion, in every period [7]. A second group, pooling records from 10 centres in the United Kingdom, Israel, Poland, Italy, France, Spain and Türkiye, compared 140 pregnancies affected before 2023 with 175 affected in 2023 and 2024. The proportion of babies who died was 6.4% before and 8.0% during the surge, a difference small enough to be chance [11]. Laboratory studies of the outbreak virus found no new or unusual strain [6].

So what did change? Two things. First, infections were being found earlier in pregnancy — the share picked up before 20 weeks rose from about a third to over half [7] — and earlier infection is the riskier kind. Second, the babies who needed transfusions needed more of them. Before 2023, about one in five transfused babies needed a second transfusion. During the surge it was nearly half, and more than a fifth needed three or more. The anaemia also took longer to settle — a median of 28 days rather than 17 [11]. That is not a sign of a nastier virus so much as a consequence of catching infections earlier, when there is more of pregnancy left for the baby to get through before the infection burns itself out.

Does every affected baby need a transfusion? This is the most interesting finding for families, and it comes from a specialist fetal medicine unit at University College London Hospitals. During the 2024 epidemic they saw 21 women. Thirteen of them — nearly two-thirds — turned out to have babies with no problems at all. Three babies received a transfusion. And five were simply watched very closely, including three babies who already had hydrops. All five recovered on their own and were born at full term without ever needing a transfusion [12].

Why watching can be the right choice

A transfusion before birth is a genuinely life-saving treatment, and no one is suggesting otherwise for a baby who is deteriorating. But it is a needle passed into a very small, very sick baby, and it carries real risk. One particular problem is that babies with severe parvovirus anaemia usually also have low platelets — the cells that stop bleeding. Across six earlier studies, around four in five affected babies had low platelets, and about a third had them severely low. Babies in that group are markedly more likely to die in the two days after the procedure [13]. When hydrops is present, the risk of losing the baby around the transfusion is roughly 29%, compared with around 5% without hydrops [12].

The London team's proposal is not to avoid transfusion but to take 48 hours, where it is safe to do so, to find out which way a baby is heading. They rescan two days later. If the blood-flow speed is still rising, they transfuse. If it is falling and the fluid is starting to clear — they describe the baby's abdominal wall looking "wrinkled" rather than tight as the swelling recedes — they keep watching, scanning at least twice a week until the anaemia resolves [12]. Those 48 hours also allow time to give steroid injections that help the baby's lungs if an early birth becomes necessary. This is a small series of five babies, and it is a proposal rather than a proven rule. But it reflects something families are rarely told: some babies with parvovirus anaemia get better by themselves, because the infection is temporary and the baby's own blood-making eventually catches up.

What happens after the baby is born

A neonatal unit in Catania, Italy, followed newborns of infected mothers through 2024 with blood tests, heart tracings and brain scans. Of 71 pregnancies with confirmed maternal infection, seven babies had laboratory-confirmed infection at birth and were studied in detail. Their results were reassuring. A blood marker of heart strain called troponin rose over the first week and then came back to normal by one month — but every single echocardiogram was normal, and there was no case of heart inflammation. Only one baby had a temporary irregular heartbeat. Two babies became anaemic in the first weeks and needed monitoring. Brain ultrasounds showed some small bright patches whose long-term meaning is not yet known. No baby died [14].

The practical lesson the Catania team drew was that most of these babies do not need intensive care. Their unit moved from admitting every affected newborn to managing most of them as outpatients with scheduled check-ups [14]. If your baby had parvovirus before birth, expect blood counts over the first weeks and a scheduled developmental review — not a long hospital stay.

What researchers are working on next

The honest gap is the long term. Children who had hydrops or a transfusion before birth appear to have a somewhat higher rate of developmental difficulties later on. In the largest careful follow-up, 28 children were assessed at around five years: three had significant difficulties and three had milder ones — higher than the background rate in the general population [15]. Brain scans in affected babies sometimes show changes in the cerebellum, the part of the brain that coordinates movement [16], a pattern also seen in French [17] and Israeli [18] series. Importantly, work comparing this with transfusions given for a different condition suggests the difficulties track with how sick the baby was and how many transfusions were needed, rather than with the virus itself [19].

Researchers now want three things. They want to follow the very large group of children born during the 2023–2024 wave, because it is the biggest opportunity there has ever been to answer the long-term question properly [7]. They want to test the "watch and wait" approach formally, so doctors can tell which babies will recover on their own [12]. And they want a vaccine. There is currently none, and no medicine that treats the infection; the only tools are finding it early and, when needed, transfusing [20]. The immunity gap that caused this wave will eventually re-form, and a vaccine for women of childbearing age is the one thing that would change the picture entirely [11].

If you are pregnant and have been exposed to slapped cheek, the most useful thing you can do is tell your maternity team promptly. A simple blood test will show whether you are already immune — many people are. If you are not, the scanning pathway that follows is well established, and the evidence from this outbreak is that it does its job.

References

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