The 280-day convention
The conventional estimate places a due date 280 days after the first day of the last menstrual period. With 70 elapsed days, 280 − 70 leaves 210 days, or 30 weeks. This is a calendar convention for estimating gestational age, not a prediction of the day labour will start.
Why clinical dates can differ
Cycle length, ovulation timing, uncertain period dates, assisted conception, and early clinical dating information can change the assigned gestational age. A later scan or clinician assessment may provide a different reference date; that clinical record should take priority over this browser arithmetic.
Starting from the right day
The calculation uses the first day of the last menstrual period, not the last day of bleeding and not an assumed conception date. If the date is unknown or cycles were irregular, entering a guessed number can make the precision of the result misleading.
Estimate versus delivery prediction
Birth can occur before or after the estimated date. The result does not assess fetal development, symptoms, pregnancy risk, or when to seek care. Contact a maternity-care professional for individual questions, concerning symptoms, or instructions from a care plan.
Use the care record
Use this page to check day counting or understand a stated estimate, then write down the source date and any updated due date from the care team. Do not use a calculator result to override medical advice.
Check the arithmetic in context
The 210-day result follows directly from 280 minus 70, and 210 divided by seven is 30 weeks. It is worth checking both expressions because an elapsed-day number may have been counted from the wrong period date. If care records use a revised estimated due date, retain that revision and the date it was assigned. Calendar arithmetic should support communication with the care team, never replace urgent advice or an appointment plan.
Weeks of gestation are conventionally counted from the menstrual-period date, which is earlier than conception for many pregnancies. This can seem counterintuitive but is why a 70-day elapsed value corresponds to ten gestational weeks in the example. Use the terminology and dating basis supplied by the maternity provider when sharing dates with others.
It is informational only and is not medical or professional advice.