Catching a Common Virus in Pregnancy: What Families Should Know About Treatment Before Birth

Four recent studies on an antiviral medicine, an antibody infusion, and why prenatal tests cannot always tell you whether a baby is affected

Cytomegalovirus is a very common virus that usually causes nothing worse than a mild cold — unless it is caught for the first time during pregnancy, when it can pass to the baby and sometimes affect hearing or brain development. Four studies published in the past year now show that an antiviral tablet taken by the mother substantially lowers that risk, while an older antibody treatment does not. They also show that no test done before birth can completely rule out infection in a baby.

What these four studies looked at

The studies come from four groups. A research team in Warsaw, Poland, working with colleagues in Gdańsk and in Kent in the United Kingdom, pooled every relevant study of the antibody treatment published over the past decade [1]. A team at Necker–Enfants Malades Hospital in Paris — France's national reference laboratory for this family of viruses — reviewed 561 pregnancies where the mother caught the virus early [2]. And two studies from Policlinico Umberto I at Sapienza University in Rome followed, first, 82 pregnancies through birth [3] and, second, 30 babies through their first months of development [4].

The problem these researchers were trying to solve

Cytomegalovirus, often shortened to CMV, is carried by most adults worldwide. It usually spreads through saliva and urine, which is why young children in nursery are a common source — and why pregnant women are advised not to share cutlery or cups with toddlers, and to wash their hands carefully after nappy changes.

About four to six babies in every thousand born in Europe and North America are born with the virus [5]. Most are completely healthy. But congenital CMV — meaning infection present at birth — is the single most common non-genetic cause of hearing loss in childhood, and a minority of affected babies have problems with vision, growth or brain development. The highest risk comes when a woman catches the virus for the very first time in early pregnancy: roughly one in three of those pregnancies passes the virus to the baby, and about half of the babies infected that early go on to have lasting hearing or neurological difficulties [6].

For a long time, there was very little to offer. A woman would be told she had caught the virus, given the statistics, offered close ultrasound monitoring, and left to wait. In 2005, that seemed to change: an Italian study reported that giving mothers an infusion of concentrated antibodies against CMV — called hyperimmune globulin — dramatically reduced the number of babies infected [7]. Many European hospitals began offering it. But when the treatment was later tested properly, with women randomly assigned to receive it or not, the benefit disappeared. One trial in Italy found no significant difference [8], and a larger trial across United States research centres also found none — and noted more early births among the women who received it [9].

Meanwhile a different idea was being tested: an ordinary antiviral tablet, valaciclovir, already used for cold sores and shingles, given at a high dose. A French-led study first used it to treat babies already infected in the womb [10]. Then a carefully designed trial at Rabin Medical Center in Petach Tikvah, Israel — in which neither the women nor their doctors knew who was getting the real tablets — found that valaciclovir cut the proportion of infected pregnancies from about 30% to about 11% [11]. A 2023 review pooling the available studies agreed [12]. What nobody yet knew was whether this held up in everyday hospital practice, and whether it made any difference to how children actually developed.

The antibody treatment: the evidence has run out

The Warsaw team gathered 13 studies involving over a thousand pregnancies [1]. When they combined the studies that had a comparison group, the antibody infusion reduced infection by about 27% — but the result was not statistically reliable, meaning it could easily have arisen by chance. Across all 13 studies, more than one in four pregnancies (27.2%) still passed the virus to the baby despite treatment.

The researchers were also blunt about the quality of the evidence. Most of the studies were rated as having serious flaws, and the overall certainty was judged low to very low. Combined with reports linking the infusions to problems in pregnancy [13] and the disappointing result of a European trial run across several countries [14], their conclusion was that hyperimmune globulin should not be given routinely outside of research studies. If it has been mentioned to you, it is reasonable to ask your team whether it is still their practice.

The antiviral tablet: strong results in ordinary care

The Paris study is the most reassuring of the four [2]. Since 2019, that hospital has offered valaciclovir to women found to have a recent first infection. Comparing 237 treated pregnancies with 324 untreated ones, the proportion of babies who caught the virus fell from 27% to 9% — roughly a two-thirds reduction, almost exactly matching what the earlier trial predicted.

One detail is worth knowing if you are in this situation. Blood tests can estimate roughly when a woman caught the virus, using a measurement called antibody "avidity" — essentially, how tightly her antibodies grip the virus, which increases over time. Loose-gripping antibodies mean a recent infection and a higher risk. The Paris team found that even "in-between" results, which are often treated as reassuring, carried a real risk — 14% to 22% of those pregnancies passed the virus on without treatment. Their advice is that these women should be offered the tablet too. They also found that the treatment helped even when the infection had happened shortly before conception, a group often told they need not worry.

The tablets are taken at a high dose, four times a day. In the Roman cohort of 82 pregnancies, where nearly everyone received them, no significant side effects were recorded in mothers or babies [3]. Reported problems elsewhere have generally been mild — stomach upset and headache.

Why a normal scan or a normal amniocentesis is not a guarantee

The Rome study delivers the hardest message [3]. Many women in this situation are offered an amniocentesis — a needle test that samples the fluid around the baby to look for the virus. It is a good test when it is positive: in this study a positive result almost always meant the baby was infected. But it missed a lot. Seven babies whose amniocentesis had been negative were nonetheless infected at birth, about 13% of those tested. The test correctly identified fewer than half of the infected babies.

Ultrasound scans and fetal MRI performed even less well. Strikingly, not one of the six babies who had symptoms at birth had shown anything abnormal on scans before delivery. Some of those findings — small calcium deposits in the brain — only became visible once the baby had been born and scanned directly. Earlier research pooling similar cases had also found that a negative amniocentesis does not entirely exclude infection [15].

None of this means the tests are useless. A negative amniocentesis genuinely lowers the odds a great deal, and in this study no family who had a negative result and reassuring scans went on to face a severe outcome without warning. But it does mean two things. Reassurance before birth should be described honestly as "much less likely," not "ruled out." And every baby whose mother had a first CMV infection in pregnancy should have a urine test in the first three weeks of life, whatever the scans showed — that test, not the prenatal ones, is what settles the question.

A first look at how the children are doing

The fourth study asked the question families most want answered [4]. The Rome team assessed 30 babies with confirmed congenital CMV at four to eight months old using a standard developmental assessment. Fifteen had been exposed to valaciclovir before birth; 15, cared for in earlier years before the treatment was available, had not. The exposed babies scored higher on the thinking-and-problem-solving part of the assessment — a median of 105 compared with 90, where 100 is the average for all children. Language and movement scores were the same in both groups.

This is genuinely encouraging, but it is a small, early study, and the two groups were treated in different eras, so other things may have changed alongside the medicine. The researchers describe their own findings as a hypothesis to be tested, not a conclusion. They also make a point that applies to every family here: babies born with this virus should have their development followed over time, not just their hearing, and not only if they seemed unwell at birth.

What researchers are working on next

The biggest remaining gap is not the medicine — it is finding the women who could benefit. Valaciclovir only helps if a first infection is picked up early, and not every country offers CMV blood tests routinely in pregnancy. Attention is now turning to testing newborns instead: a large programme in New York has tested babies' routine heel-prick blood spots for the virus [16], and specialists are rewriting guidance for the many babies who will be found this way and who appear entirely well [17]. For those babies, antiviral treatment after birth — studied first as an intravenous drug [18] and later as a six-month oral course for babies with symptoms [19] — is not currently recommended, and care follows international consensus guidance published in 2017 [20].

Researchers are also asking whether the antiviral should be continued for longer rather than stopped after a negative amniocentesis, whether it helps women who had the virus before pregnancy and reactivate it, and whether the early developmental benefit is still there at two years old.

If you are reading this because you have just had an unexpected blood test result, the practical summary is this: there is now a safe tablet that meaningfully lowers the chance of your baby being infected, it works best when started early, and it is worth asking about straight away. The antibody infusion is no longer supported. And whatever the scans show, your baby's urine test in the first weeks of life is the one that gives the real answer — and is worth insisting on.

References

  1. Gutowska K, Kucińska-Chahwan A, Bednarek M, et al. Efficacy of Immunoglobulin Therapy for Secondary Prevention of Congenital Cytomegalovirus Infection: A Systematic Review and Meta-Analysis. Open Forum Infect Dis. 2026;13(7):ofag431. doi:10.1093/ofid/ofag431 ↩
  2. Fourgeaud J, Barry M, Bourgon N, et al. Cytomegalovirus Vertical Transmission Rate According to IgG Avidity Value and Valacyclovir Treatment of Maternal Primary Infection in the First Trimester of Pregnancy. Clin Infect Dis. 2025;81(5):e417–e423. doi:10.1093/cid/ciaf437 ↩
  3. Arcieri F, Vasta A, Sorrenti S, et al. Antenatal Imaging and Neonatal Outcome in Infants with Congenital Cytomegalovirus Infection: The Effect of Valaciclovir. J Clin Med. 2026;15(2):809. doi:10.3390/jcm15020809 ↩
  4. Arcieri F, Vasta A, Volpe G, et al. Impact of Maternal Valaciclovir Therapy on Early Neurodevelopment in Congenital CMV Infection: A Retrospective Pilot Study. Children (Basel). 2026;13(4):566. doi:10.3390/children13040566 ↩
  5. Ssentongo P, Hehnly C, Birungi P, et al. Congenital Cytomegalovirus Infection Burden and Epidemiologic Risk Factors in Countries With Universal Screening: A Systematic Review and Meta-analysis. JAMA Netw Open. 2021;4(8):e2120736. doi:10.1001/jamanetworkopen.2021.20736 ↩
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  15. Chatzakis C, Sotiriadis A, Dinas K, Ville Y. Neonatal and long-term outcomes of infants with congenital cytomegalovirus infection and negative amniocentesis: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2023;61(2):158–167. doi:10.1002/uog.26128 ↩
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