Pregnancy weeks usually count from the first day of the last menstrual period, not from conception. Pregnancy months are approximate because calendar months are longer than four weeks. Use weeks for due-date math, appointment timing, and clinician documentation.
Source basis: This guide cross-checks the practical answer against ACOG, Merck Manual and the full references listed below.
Pregnancy dating usually counts from the first day of the last menstrual period. That can make week, month, and trimester math feel strange, because the count begins before conception usually happens.
The useful split is simple: use weeks for medical timing and due-date calculators, and treat months as approximate plain-language labels.
Why pregnancy starts at the last period
Pregnancy weeks are usually counted by gestational age. That count starts on the first day of the last menstrual period because it is a date many people can identify more reliably than ovulation or fertilization.
This is why the math can feel off. Merck Manual explains that pregnancy dating is usually estimated as 40 weeks after the first day of the last menstrual period, while fertilization typically happens about two weeks after that point.
Weeks and trimesters
Merck Manual describes the first trimester as week 0 through 13 weeks and 6 days, the second trimester as week 14 through 27 weeks and 6 days, and the third trimester as week 28 through delivery.
Due dates are estimates. ACOG guidance explains that dating can use last menstrual period information and ultrasound findings, especially early in pregnancy.
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Why months do not line up perfectly
A calendar month is not exactly four weeks, so pregnancy month charts are approximations. They are useful for plain-language conversations, but weeks are the cleaner unit for appointments, fetal-development notes, and due-date tools.
A practical month map is weeks 1-4 for month 1, 5-8 for month 2, 9-13 for month 3, 14-17 for month 4, 18-22 for month 5, 23-27 for month 6, 28-31 for month 7, 32-35 for month 8, and 36-40+ for month 9.
What date should you use?
Start with the first day of your last menstrual period when you know it. If cycles are irregular, the date is uncertain, conception happened through fertility treatment, or an early ultrasound gives a different estimate, use the date your clinician documents.
For everyday planning, Doola calculators can translate LMP or due date into weeks and months. For medical decisions, appointment timing, or a discrepancy between tools, use the dating your clinician gives you.
References
Source-cited references used for this article. Open the original guidance when you want the public-health details behind the summary.
