The Persona, which has the backing of the Catholic Church, has been bought by more than 100,000 women since going on sale at Boots last October.
It works by measuring hormone levels in urine to establish when a woman is at her most fertile.
The device includes a computerised monitor, which displays a green light when it is safe to have sex and a red one when the user is fertile.
Manufacturers Unipath told BBC1's Watchdog Healthcheck programme last night that they knew of at least 450 pregnancies. The company has also recently reduced its claimed success rate for the product from 95 per cent to 94 per cent.
Persona was marketed as an alternative to the condom - but family planning statistics give condoms a 98 per cent success rate.
The chairman of Unipath, Leendert Staal, last night defended the device.
"No method of contraception is 100 per cent reliable and therefore there are pregnancies associated with any method used," he said.
"The reliability for Persona is 94 per cent, which means that for every 100 women using Persona for one year, six would be expected to become pregnant as a result of Persona not identifying the fertile phase correctly."Reuse content