Reverse Due Date Calculator

Determines the conception date from the estimated due date or from last period to find out when a baby was conceived or when to try for a baby with a particular birth date.

Refer to the text below the tool for more information about calculating the conception date.


This reverse due date calculator retrieves the likely intercourse/conception date based on either the estimated due date obtained after the dating ultrasound or from data about the last menstrual cycle.

It can help you either by going in time to see the likely date on which your baby was conceived or to help you plan your pregnancy, if for example, you desire to plan it and birth for a particular period of the year, so you know when you should increase intercourse to try to conceive.


The reverse due date is based on estimates that account for certain rules regarding the menstrual cycle and pregnancy length:

  • Due date can either be estimated at 280 days from last menstrual period (LMP) or calculated from measurements (crown to rump) taken during dating ultrasound;
  • Conception is estimated to have taken place around ovulation;
  • For a regular 28-day cycle and 40-week pregnancy (21% of pregnancies) there are 14, respectively 266 days, before and after ovulation;
  • By going back 266 days from the due date, it is considered ovulation, thus likely conception can be estimated.

Note: Use this tab if Due Date is known (if unknown go to Method 2).

Due Date
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Note: By this method you can calculate the reverse due date based on last menstrual cycle details.

First Day of Your Last Menstrual Period
Menstrual Cycle Length
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


 

About the reverse due date

The reverse due date retrieves the likely conception date based on either the estimated due date obtained after the dating ultrasound or from data about the last menstrual cycle.

This conception calculation can help you either by going in time to see the likely date on which your baby was conceived or to help you plan your pregnancy, if for example, you desire to plan it and birth for a particular period of the year, so you know when you should increase intercourse frequency to try to conceive.

The reverse due date is based on data related to certain rules regarding the menstrual cycle and pregnancy length:

  • Due date can either be estimated at 280 days from last menstrual period (LMP) or calculated from measurements (crown to rump) taken during dating ultrasound;
  • Conception is estimated to have taken place around ovulation;
  • For a regular 28-day cycle and 40-week pregnancy (21% of pregnancies) there are 14, respectively 266 days, before and after ovulation;
  • By going back 266 days from the due date, it is considered ovulation, thus likely conception can be estimated.
 

Menstrual cycle basics, ovulation and conception

During their fertile period, females start a new menstrual cycle every 28 days, in average, with lengths varying from 26 days to 36 days. Each cycle consists of three different phases, controlled by the endocrine system: follicular phase, ovulation and luteal phase.

The period bleeding marks the start of the follicular phase. As a rule of thumb, it is considered that whilst the follicular phase can vary in length, the luteal phase is more or less close to 14 days, regardless of the menstrual cycle length. So for a menstrual cycle of average 28 days, ovulation occurs on approximately the 14th or 15th day.

Depending on egg and sperm viability, the fertile window, hence the time period when a baby can be conceived during a menstrual cycle, is considered to be between 5 days before ovulation and up to 2 days after.

To help with tracking ovulation, you can either monitor the cervical mucus or start charting the basal body temperature.

For ease of reference, the likely conception date is estimated as closest to the ovulation date for the specified menstrual cycle.

If the calculation is done by going back from the due date (either estimated or dated on ultrasound), then 266 days are subtracted (considering a full-term pregnancy of 40 weeks).

Dating the pregnancy from the date of the last menstrual period or following the dating ultrasound, doesn’t only help obtain the due date and give you a date for birth but will also provide the gestational age of the baby and the pregnancy weeks.

Bear in mind that the pregnancy weeks start from the LMP and not from the conception date, so technically, the first 2 weeks of pregnancy, you are not actually pregnant and in some dating conventions you may find your pregnancy week as 11+2 rather than 13, to account for the 2 week difference.

 

References

Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the Conceptus and loss of pregnancy. New England Journal of Medicine. 1999; 340 (23):1796–1799

Marieb EN, Hoehn K. Human Anatomy and Physiology, 5th ed. 2013Pearson

Kullinger M. et al. Discrepancy between pregnancy dating methods affects obstetric and neonatal outcomes: a population-based register cohort study. Sci Rep. 2018; 8: 6936.


Specialty: Obstetrics Gynecology

System: Reproductive

Article By: Denise Nedea

Published On: June 16, 2020

Last Checked: June 16, 2020

Next Review: June 16, 2025