I've gained 18 pounds so far and I'm 6 months pregnant, is this enough weight?

If you were 5'7 and weighed 115-117 pounds to begin with then you would be considered underweight, so you should gain between 28-40 pounds during your pregnancy so it wouldn't really be healthy for your baby if you weighed less than 140 at the end of your pregnancy. Stop worrying so much about the weight you are putting on. You are putting it on for a good reason and you will have plenty of time to take it off after the baby is born.