If high blood pressure is detected late in pregnancy, labour is usually induced so the child is born as soon as possible.

London - More than 15 000 pregnant women a year are needlessly having labour induced because their blood pressure is measured incorrectly, experts have warned.

High blood pressure is a major red flag during pregnancy as it may be a sign of pre-eclampsia – which can threaten the lives of mother and child.

If high blood pressure is detected late in pregnancy, labour is usually induced so the child is born as soon as possible.

But experts at St George’s Hospital in London have warned that many women are having inducements unnecessarily because their blood pressure has been raised artificially by the stress of being in hospital. This problem – "white coat syndrome" – could be averted by measuring blood pressure at home.

The team has developed a blood pressure cuff that connects to a mobile phone and allows women to accurately test their levels at home.

Studies suggest around 25 to 30 percent of these – 15 000 a year – suffer from "white coat syndrome".

Pre-eclampsia is caused by reduced blood flow in the placenta, which restricts the flow of oxygen and nutrients to the baby and prevents growth.

Although experts know the warning signs – protein in the urine, headaches and swollen ankles – these symptoms often appear in healthy pregnancies.

This means it is often missed until blood pressure soars, at which point labour is likely to be induced.

Inducing childbirth can be an unpleasant procedure, with labour triggered either by rupturing a woman’s membranes or by inserting artificial hormones.