Is this the right tool?
- Best for
- Estimating current gestational age and trimester on a selected as-of date from the first day of the last menstrual period.
- Choose another tool when
- Use Due Date Calculator when you only need a due date from LMP, conception, or IVF transfer; use Pregnancy Conception Calculator for a broad conception window.
Decision context
Keep calendar dating separate from clinical dating
Pregnancy weeks are conventionally counted from the last menstrual period, which is not the same thing as a confirmed conception date or a clinical assessment.
Use the week count to organize a question, not to replace prenatal assessment or an established clinical due date.
LMP and conception are different anchors
Gestational age is commonly dated from the first day of the last menstrual period, which precedes conception in a typical cycle.
Clinical dating can refine an estimate
Early ultrasound and clinical information may confirm or revise dating in ways a calendar calculator cannot.
Illustrative scenario
A person with an uncertain last menstrual period gets a date range from the calculator. They bring that uncertainty to a prenatal appointment rather than treating the midpoint as confirmed.
Method notes and further reading
Pregnancy Calculator links these resources for keep calendar dating separate from clinical dating. Use the original material when a decision depends on a current rule, a personal circumstance, or a professional standard.
- American College of Obstetricians and Gynecologists: Methods for Estimating the Due DateClinical dating guidance. This site does not provide prenatal care, ultrasound dating, or individualized medical assessment.Reviewed July 2026.
What this calculator does
The calculator counts calendar time from the entered last menstrual period and expresses it in weeks and days. It can help organize questions for a prenatal appointment.
When to use it
Use it to understand the conventional gestational-age count, prepare a timeline question, or compare an estimated date with information already provided by a clinician.
Inputs explained
- Last menstrual period: the first day of the last period used for pregnancy dating estimates.
- As-of date: the date on which pregnancy age is estimated.
Formula or method
The tool counts elapsed days from the first day of the last menstrual period. Clinical dating can use menstrual history, ultrasound findings, and other information that this calculator does not assess.
Worked example
If the last menstrual period date is uncertain or cycles are irregular, changing the input date can change the whole result. That sensitivity is a reason to discuss uncertainty with a clinician.
How to interpret the result
The displayed week is a calendar estimate. It does not confirm conception, fetal development, viability, pregnancy location, or whether an estimated due date should be changed.
Practical checks before using the result
- Enter the first day of the last menstrual period, not an assumed conception date, unless a clinician has given a different dating basis.
- Seek timely medical care for pregnancy concerns, pain, bleeding, or urgent symptoms rather than relying on a date tool.
Common mistakes
- Treating a calendar estimate as confirmation of a pregnancy's clinical age.
- Changing an established clinical due date because a simple date count differs.
Limitations and disclaimers
These results are general estimates only and are not medical advice. Individual health, body composition, activity, menstrual history, pregnancy dating, medications, and clinical findings can change what is appropriate.
Related calculator context
Use Due Date and Pregnancy Conception calculators only for related date estimates. Clinical ultrasound and prenatal care have a different role.
Related glossary terms
These plain-English definitions can help you check the terms used in this calculator before relying on the result.
Frequently Asked Questions
Why can a last-menstrual-period estimate differ from clinical dating?
Gestational age is commonly dated from the first day of the last menstrual period, which precedes conception in a typical cycle. The displayed week is a calendar estimate. It does not confirm conception, fetal development, viability, pregnancy location, or whether an estimated due date should be changed.
Which date should be used as the reference for this estimate?
Enter the first day of the last menstrual period, not an assumed conception date, unless a clinician has given a different dating basis. Seek timely medical care for pregnancy concerns, pain, bleeding, or urgent symptoms rather than relying on a date tool.
When does pregnancy timing need clinical rather than calendar guidance?
These results are general estimates only and are not medical advice. Individual health, body composition, activity, menstrual history, pregnancy dating, medications, and clinical findings can change what is appropriate.