18-22 weeks is 5 months :)
Going by 4 week intervals you would be 5 months and 1 week. Using standard 30-31 days you would be just about 5 months.
With 4 week intervals pregnancy is dated to 10 months, while 30-31 days you use 9 calendar months. Either way it all equals out the same. Most people will use 4 week intervals to determine months, but when telling someone how far along they are many more will just use weeks instead of months.