Several other passengers, from both vehicles, were transported from the scene by road ambulance.
One of the passengers was a child.
Two of the four patients have now been discharged from Waikato Hospital, including the child.
The pregnant woman, and a man are both stable in wards.
Traffic was diverted around the crash on Tirau Rd, Karapiro.
The police serious crash unit is investigating but say it is too early to determine the cause of the crash.
Six crashes were reported in the space of three hours yesterday, police said.
An hour after the Karapiro crash, there were two further crashes in the district -- one in Matamata where there were no injuries reported, and one in Huntly, where there were minor injuries reported.
"While not as serious as the Karapiro crash, all crashes have the potential to cause devastation to families," police said.
Then, at 7.30pm, a vehicle using the diversion around the Karapiro incident, crashed on Whitehall Rd.
This was a single car crash, and the driver was taken from the scene before police arrived.
"Safety is the message and adjusting your driving to suit the conditions," police said.
"As the festive season approaches everyone using the roads needs to ensure they are safe drivers ... we want to make sure that all of the family is around the table to celebrate Christmas."