Gender blood tests

I had the blood tests done for Down’s syndrome etc.. which also gives you the gender.. I haven’t told anyone and i know the gender but all my co workers are telling me to wait till 20 weeks to announce till it can be confirmed by ultrasound.

The doctor only said be cautious when it comes up girl. Which it came up boy, cause Incase they would happen to mix maternal dna with babies.

How likely is it to be wrong?