Pregnancy weeks start with the first day of your last menstrual cycle. Months are measured by weeks and not by regular calendar months. For example August has 31 days but 4 weeks at 7 days each is only 28 days, thus making August longer then a pregnancy month. So at 4 weeks in a pregnancy month time 40 weeks you are pregnant for 10 months but it only comes out to 9 calendar months. Sometimes its a little confusing, so I listed it out for you below................
Days 1 - 28 = Month 1
Day 1 - 7 = week 1
Day 8 - 14 = week 2
Day 15 - 21= week 3
Day 22 - 28= week 4 or 1 month
Days 29 - 56 = Month 2
Day 29 - 35= week 5 or 1 months 1week
Day 36 - 42= week 6 or 1 months 2 weeks
Day 43 - 49= week 7 or 1 months 3 weeks
Day 50 - 56= week 8 or 2 months
Days 57 - 84 = Month 3
Day 57 - 63= week 9 or 2 months 1 weels
Day 64 - 70=week 10 or 2 months 2 weeks
Day 71 - 77=week 11 or 2 months 3weeks
Day 78 - 84=week 12 or 3 months
Days 85 - 112 = Month 4
Day 85 - 91 =week13 or 3 months 1 week
Day 92 - 98 =week14 or 3 months 2 weeks
Day 99 - 105 =week15 or 3 months 3 weeks
Day 106 - 112=week 16 or 4 months
Days 113 - 140 = Month 5
Day 113 - 119=week17 or 4 months 1 week
Day 120 - 126=week18 or 4 months 2 weeks
Day 127 - 133=week19 or 4 months 3 weeks
Day 134 - 140=week20 or 5 months
Days 141 - 168 = Month 6
Day 141 - 147=week21 or 5 months 1 week
Day 148 - 154=week22 or 5 months 2 weeks
Day 155 - 161=week23 or 5 months 3 weeks
Day 162 - 168=week24 or 6 months
Days 169 - 196 = Month 7
Day 169 - 175=week25 or 6 months 1 week
Day 176 - 182=week26 or 6 months 2 weeks
Day 183 - 189=week27 or 6 months 3 weeks
Day 190 - 196=week28 or 7 months
Days 197 - 224 = Month 8
Day 197 - 203=week29 or 7 months 1 week
Day 204 - 210=week30 or 7 months 2 weeks
Day 211 - 217=week31 or 7 months 3 weeks
Day 218 - 224=week32 or 8 months
Days 225 - 252 = Month 9
Day 225 - 231=week33 or 8 months 1 week
Day 232 - 238=week34 or 8 months 2 weeks
Day 239 - 245=week35 or 8 months 3 weeks
Day 246 - 252=week36 or 9 months
Days 253 - 280 = Month 10
Day 253 - 259=week37 or 9 months 1 week
Day 260 - 266=week38 or 9 months 2 weeks
Day 267 - 273=week39 or 9 months 3 weeks
Day 274 - 280=week40 or 10 months