Hundreds of babies and small children in the Herts Valleys are not registered with a doctor, and could potentially be missing life-saving health checks.
Doctors have warned children could be at risk of missing out on key vaccinations and checkups because their parents haven't signed them up with a surgery.
NHS figures show there are 38,768 newborns and children under five registered with GP practices in the NHS Herts Valleys Clinical Commissioning Group area.
However, the latest ONS figures put the number of children aged four and under in the CCG's area at 39,117.
This means around 350 children are not on a GP's register.
Across England, more than 123,000 children - 4% of the population - are not registered with a doctors' surgery, the figures suggest.
Professor Russell Viner, president of the Royal College of Paediatrics and Child Health, has urged parents to ensure their children are registered.
She said: “I find it deeply concerning that so many children appear to be unregistered with a GP, at a time where we are seeing an emergence of preventable and deadly diseases such as the measles, record levels of childhood obesity and its associated conditions, and with 34% of all child deaths in the UK considered avoidable – the vast majority of which are in infancy."
The NHS offers regular health checks for babies until they are two years old, to monitor their development.
They are also given a personal child health record, known as a red book, in which parents and health professionals keep a log of their vaccinations and measurements.
In 2017-18, just 87% of children in England had received both the recommended MMR jabs - which protects against measles, mumps and rubella - by the time they turned five.
At least 95% coverage is needed to lower the risk of an outbreak, according to the World Health Organisation.
A monthly snapshot of GP patients was taken on January 1, while the most recent population estimates are for mid-2017.
This means the number of children not on a GP register is an estimate.
However, the number of GP patients is generally higher than the actual population, as many patients are on multiple registers as a result of moving house, or are not removed after they die.
Professor Viner continued: “Registering a child with a doctor can be life saving or at the very least, life changing.
"It will mean parents get important vaccination and health check reminders, their child’s weight and development monitored and concerns acted upon quickly.
"If required, access to specialist NHS healthcare can be arranged.
"I urge all parents to register their child with their local GP and, if unsure, visit nhs.uk to find out more.”