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Pregnancy Calculator

This pregnancy calculator estimates gestational age, due date, conception timing, and last menstrual period (LMP) using one known clinical milestone and the patient's menstrual cycle length. It is designed for use in early pregnancy assessment, particularly when ultrasound is not yet available. In outpatient, emergency, and primary care settings, the tool provides fast and standardized estimates to support care planning and patient counseling.

Users can enter whichever detail is known, such as LMP, estimated gestational age (EGA) as of a specific date, estimated date of conception (EDC), or estimated due date (EDD), along with the cycle length. The calculator then determines the other three values using standard obstetric formulas. This structure allows for flexibility across clinical workflows and aligns with typical patient recall patterns.

Although ultrasound is preferred for dating confirmation, especially in the first trimester, the calculator provides a reliable starting point. Adjusting for cycle length helps refine estimates by aligning them with actual ovulation timing when available.

Pregnancy Calculator

Pregnancy Calculator

Cycle Length (Days)

Weeks

Days

Calculate

Value

Value

Value

Value

Pregnancy Calculator Formulas

This pregnancy calculator calculates four key clinical dates based on one input and the patient's menstrual cycle length. These include gestational age, due date, conception date, and LMP. The outputs are derived using standard obstetric relationships that account for cycle variability.

Estimated gestational age (EGA) is calculated as the number of days or weeks since the LMP. If another value is provided, such as EDD or EDC, the calculator first determines the LMP and then calculates gestational age as of today or any specified date.
• Estimated gestational age (EGA) = Days since LMP

Estimated date of conception (EDC) is set at 14 days after the LMP for a 28-day cycle. When the cycle length is different, the calculator shifts ovulation accordingly. For example, in a 32-day cycle, conception is assumed to occur on day 18.
• Estimated date of conception (EDC) = 14 days since LMP

Estimated due date (EDD) is calculated by adding 280 days to the LMP, following Naegele’s Rule. When the cycle length differs from 28 days, the difference is added or subtracted to adjust the total gestational interval. For instance, a 25-day cycle results in an EDD of LMP plus 277 days.
• Estimated date of conception (EDD) = 1st day of LMP + 280 days

Last menstrual period (LMP) is estimated when EGA, EDD, or EDC is provided.

This method supports a wide range of clinical scenarios. The tool allows users to start with whichever value is most reliable or available and produces a complete set of pregnancy estimates without switching tools.

Limitations of This Pregnancy Calculator

This calculator relies on typical menstrual and ovulation patterns to generate estimates. In patients with irregular cycles, recent hormonal contraceptive use, or uncertain LMP, the values may be less accurate. Ultrasound, especially in the first trimester, should be used to confirm gestational dating in these cases.

It does not adjust for specific fertility treatments, such as embryo transfer, or for multiple gestations. Clinical situations that require precise timing, such as high-risk pregnancies or scheduling of time-sensitive interventions, should rely on ultrasound and documented reproductive data when available.

For routine care, the calculator offers a practical and fast way to establish baseline pregnancy dates and align follow-up care with gestational timelines.

Learn more on the BackTable OBGYN Podcast

BackTable is a knowledge resource for physicians by physicians. Get practical advice on the Pregnancy Calculator and how to build your practice by listening to the BackTable Podcast.

Laparoscopy in Pregnancy: Key Considerations for Surgeons
Ep 79 Laparoscopy in Pregnancy: Key Considerations for Surgeons with Dr. Craig Sobolewski
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References

[1] Cunningham, F. G., Leveno, K. J., Bloom, S. L., Dashe, J. S., Hoffman, B. L., Casey, B. M., & Spong, C. Y. (2018). Williams Obstetrics, 25th Edition. McGraw-Hill Education.
[2] Committee on Obstetric Practice. (2017). Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 129(5), e150-e154.
[3] Savitz, D. A., Terry, J. W., Dole, N., Thorp, J. M., Siega-Riz, A. M., & Herring, A. H. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology, 187(6), 1660-1666.

Disclaimer: The Materials available on BackTable.com are for informational and educational purposes only and are not a substitute for the professional judgment of a healthcare professional in diagnosing and treating patients. The opinions expressed by participants of the BackTable Podcast belong solely to the participants, and do not necessarily reflect the views of BackTable.

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