Pregnancy is dated in weeks from the last menstrual period (LMP), not by calendar months â which is why the blogs disagree. Two conventions are in use: in the lunar-month system (28 days each), which most Indian OB textbooks follow, weeks 25 to 28 make up the 7th month. In calendar months, it works out to roughly weeks 27â30. Either is acceptable; your gynaecologist will always track pregnancy by weeks, not months.
Next Steps
If you already have a due date from your obstetrician, count backwards 40 weeks to find the LMP, and every week can be mapped to a month either way. From month 8, scans typically move from monthly to fortnightly, so the visit schedule itself confirms the last stretch.