Women expecting to give birth have been advised to arrange their own transport to hospital as paramedics prepared to walk out for 12 hours from 11am.
Lewisham and Greenwich NHS Trust said that delays meant that clinicians could not "guarantee a quick journey into hospital should you need it during an emergency for you and your baby".
And St George's NHS Trust warned patients that an ambulance "may not be able to attend to you at home in an emergency or in labour".
"Make sure you have plans to get to hospital using private transport like car or taxi," the trust said.
Chelsea and Westminster NHS Trust said that the strikes meant there was “no guarantee that an ambulance or paramedic will be able to come to your home in the event of a homebirth complication”.
They added: “The days around this date may also be affected and ambulances may take longer to reach homes than usual.”
Other NHS trusts in the capital warned that their emergency departments would be “extremely busy”.
Barking, Havering And Redbridge University Hospitals NHS Trust said the strikes would pile "further pressure on our already busy hospitals, particularly our emergency departments (EDs)", adding: "Anyone coming to our departments who does not need emergency care will face extremely long waits".
Dr Mark Harmon, an A&E doctor Clinical Entrepreneur at eConsult, said that "already burnt-out staff and patients in need of critical care" would suffer the most during Wednesday’s industrial action.
"The sad reality is that even without the ambulance strikes, A&E departments are no longer just cracking at the seams. We’re at breaking point."
Talks between Health Secretary Steve Barclay and health unions failed to yield a settlement on Monday but union officials today indicated they would consider a one-off payment to end the strikes.
Christina McAnea, general secretary of Unison, told BBC Radio 4's Today programme, If it was a really good one-off payment, if it was a really good offer for next year that was going to be backdated, then we would look at any of them."
Unison's head of health, Sara Gorton, acknowledged that the union had stepped up its action by including call handlers in Wednesday’s walkout.
“The message is there that we’re, you know… there’s escalation room, but we’re equally willing to talk, we want a resolution,” she said.