I agree with 3boys - companies like Harmony are primarily looking for chromosomal abnormalities, but can tell gender too and are very reputable and accurate.
The companies you can buy a 'fingerprick' home test from online, just for gender, have a very bad reputation and you only have to google their reviews to see how many wrong results they give out!
I was very tempted to get one myself because I couldn't face waiting for my early gender scan, but I'm so glad I didn't waste my money in the end after reading about them online.