The Nursing and Midwifery Council have said more pregnancies were being seen in older women, immigrants and people with health problems across the UK.
It said these cases often required more attention pre-birth and during labour, which added to the pressure on an already under-staffed service.
NMC officials spoke out as the regulator published its annual report based on feedback from midwife supervisors – senior midwives who oversee community and hospital teams.
Midwives believe another 5,000 midwives are needed immediately to boost the workforce close to the 40,000 needed to provide a satisfactory service.
The government in England has responded by promising to invest an extra £330m in maternity services to boost staff numbers by 4,000 over the next four years, while Wales and Scotland are also aiming to expand the workforce.
A Department of Health spokesman said the NHS needed to think carefully about how it was going to deploy the extra staff.
“We recognise that with the rising birth rate and increased complexities of pregnancies more midwives will be required in some areas.”
But he added: “The UK remains one of the safest countries in the world to have a baby.”