The woman was travelling from Aberdeen to Gatwick when she fell ill suddenly on Thursday morning.
The easyJet pilot decided to divert the plane to the nearest airport and landed in Birmingham at about 11:15am.
Passenger Shona Littlejohn was travelling to London to celebrate her 50th birthday when the mid-air drama unfolded.
She said: “A woman on the plane who was expecting collapsed and needed medical attention.
“After they made an announcement asking if there were any doctors or nurses on board, I saw a woman going up to help.
“They then weren’t too long in announcing that we were diverting to the nearest airport.
“I asked the air hostess as we were getting off how she was, and they all seemed to thing she was going to be okay.”
There were 125 passengers on board the airbus A319, which had departed from Aberdeen International Airport just after 10am.
Ms Littlejohn said the passengers all appeared to be understanding.
She added: “It was all handled quite well be the airline.”
The diverted flight landed at Gatwick Airport at 1pm.
An easyJet spokeswoman apologised to all passengers and stressed that safety was their priority.
She said: “We can confirm that flight EZY872 from Aberdeen to London Gatwick diverted to Birmingham Airport due to a passenger on board requiring urgent medical assistance.
“The flight continued on to London Gatwick.
“We apologise for any inconvenience experienced due to the diversion and resultant delay.”