The world's biggest vaccine maker by sales did not give further details on why it paused the trial, dubbed "GRACE," as well as two other studies, but said a trial of its experimental RSV vaccine for adults aged 60 and over would continue.
GSK's London-listed shares were 0.6% lower at 1,570.6 pence by 0833 GMT, having slipped as much as 1.5% in earlier trade.
RSV is a leading cause of pneumonia in toddlers and the elderly, but vaccine development has been plagued by setbacks for decades. However, many drugmakers are working to bring a candidate to the market over the next few years.
RSV vaccines are seen as an opportunity for GSK to deliver on pledges to boost its drug development success rate, where it has long lagged industry peers. GSK's shot for pregnant women was aimed at conferring immunity to protect unborn children.
Its vaccine for the elderly, another major vulnerable population, has been regarded by analysts as a potential big RSV market, with trial results expected this year.
Liberum analyst Alistair Campbell said that while the maternity trial pause was "disappointing," the "big opportunity" for RSV vaccines was in the elderly, with potential peak sales of about $2 billion.
RSV causes symptoms similar to a cold and can also cause bronchiolitis, leading to about three million hospital stays for children under five per year.
Swedish Orphan Biovitrum's Synagis, a monthly shot, is currently the only preventive treatment against RSV in high-risk infants.
J&J, Sanofi, Moderna and AstraZeneca are among drugmakers racing to get an RSV therapy or vaccine approved.
On Thursday, AstraZeneca's candidate for infants was approved for a speedy review by European regulators.
Read also: GSK Pharma net profit falls 3.9 percent to Rs 150.3 crore in Q3
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