Births at home and at midwife-led units have been suspended in some areas due to the "significant pressure" the ambulance service is facing.
The East Sussex Clinical Commissioning Group (CCG) said immediate changes had to be made "to ensure that all births can take place as safely as possible".
It apologised to expectant mothers for any "distress" the changes might cause.
The Medway NHS Foundation Trust has also confirmed a suspension of "at least three weeks".
It said home births were safe for many women but there was always the risk of complications.
"Access to an ambulance is vital for the small number of women who experience an emergency," it added.
The trust apologised for any disappointment but stressed it was taking the step "because we want to keep you safe when you have your baby".
In a letter to NHS trusts in its region, South East Coast Ambulance Service (Secamb) spoke of the "current operational pressures" it was facing.
"We need to reduce the amount of planned activity that may require emergency ambulance transfer," it said.
East Sussex CCG explained the ambulance service was under "significant pressure".
It said there would be a "higher degree of risk" when "the ambulance service will be stretched and response times may be delayed".
But it reassured expectant mothers that its home birthing teams and unit midwives would still "continue to provide care".