It happens; don't beat yourself up. But do be diligent to try to remember. I bought one of those pill cases which has a compartment for each day of the week. I fill it up every Sunday after I've used the week's supply of vitamins (I take prenatal and folic acid as well as one other pill) and then I keep it somewhere where I will see it and won't forget to take it.
It's not horrible. I'm 18 weeks as well and I sometimes forget for a couple of days. With my first pregnancy I really didn't take any vitamins because I was so sick and I just told my midwife I couldn't take them. She said as long I ate healthy foods, it was okay. But this time around, I just want to be diligent about it, I have only forgotten a few times, but it's not going to hurt the baby in any way. Maybe try using a pill dispenser? And put it somewhere where you will definitely see it everyday. That could help.