Is it common to experience swelling feet in week 18 of your pregnancy?

Yes, it is quite common. This is known as Edema (Swelling in Feet & Ankles). http://www.whattoexpect.com/pregnancy/symptoms-and-solutions/edema.aspx As your body tissues accumulate fluid (your pregnant body requires extra fluid to support you and your baby), you may notice swelling in your ankles and feet (fluid tends to pool there thanks to the law of gravity). Reduce water retention in your feet by avoiding standing or sitting for long periods — and try to keep your legs elevated when you can. Excessive swelling can be one sign of preeclampsia – but when it is, it's accompanied by a variety of other symptoms (such as elevated blood pressure, rapid weight gain and protein in the urine).