Emma Cave was 30 weeks pregnant when she went to her GP for a whooping cough vaccination, which is routinely given during pregnancy.
The 25-year-old was given a jab and went home, none the wiser that she'd been given the wrong vaccination.
She only found out about the mistake the next day when the surgery called to reveal their error – and that they couldn't guarantee her unborn baby wouldn't be harmed.
Emma, from Pudsey, West Yorkshire, told her local paper: "I felt sick to the stomach when they told me what had happened.
"It was all a complete shock as you expect nurses and doctors to know what they are doing, and not make such silly, potentially serious mistakes.
"It wouldn't have been so bad had they been able to fully assure me there and then that there was no danger of harm to my baby, but they couldn't."
Emma said she suffered months of worry following the blunder and had to wait until her son Harrison was six-weeks-old before she was finally told for certain he hadn't been harmed.
NHS Choices says the MMR jab is prepared with weakened live viruses so doctors advise it isn't suitable for women who are pregnant or who might become pregnant within a month after being vaccinated.
Emma took legal action and the GP surgery admitted the mistake and apologised.
Her solicitor Kay Barnes said: "It was a mother's nightmare, an unfortunate incident involving negligent treatment which could have been completely avoided had proper care been taken."