I think someone pointed out earlier that only one month is actually only 4 weeks. People who say pregnancy is 9 months are going by calendar months, not lunar months (which are actually 29 days, not 4 weeks).
The problem with counting months is that a month is not a fixed period of time. One day is 24 hours, fixed. One week is 7 days, fixed. One month varies from 28 to 31 days and depends on your definition of the word "month".
If you're looking at 9 calendar months, those 9 months will last anywhere from 273 to 276 days, with an average of 273.75 days (not counting leap years). That means those 9 months will last anywhere from 39 weeks to 39 weeks and 3 days, which is pretty close to either 40 weeks (standard counting) or 38 weeks (gestational counting).
Bottom line, at least to me, is that months are a meaningless number to talk specifics with, since we all use them differently and they're of varying lengths. So, why sweat the exact number of months when we have better counting methods for when it really matters. No casual acquaintance who asks how far along you are will see any difference between 5 and 6 months anyway.