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Reverse Due Date Calculator to Calculate Conception Dates

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Reverse Due Date Calculator Sign
Reverse Due Date Calculator

This extremely popular conception date calculator will calculate what date and day of the week your baby was conceived on based on your pregnancy due date, ultrasound weeks and days, or the 1st day of your last menstrual period (LMP). Also includes estimated dates for the range of days the pregnancy-causing "event" likely occurred within. Plus, you can also select any birth date -- meaning you can use the calculator to answer questions such as ...

  • When was I conceived?
  • When did I conceive my baby?
  • When to get pregnant to have a baby in April (or any other month)?

The Learn section includes:

  • How to calculate conception date (Conception Calendar Calculator).
  • How to plan baby birth date (Birth Month Conception Range Chart).
Learn More

Selected Data Record:

A Data Record is a set of calculator entries that are stored in your web browser's Local Storage. If a Data Record is currently selected in the "Data" tab, this line will list the name you gave to that data record. If no data record is selected, or you have no entries stored for this calculator, the line will display "None".

DataData recordData recordSelected data record: None
Event:Event:Event to base on:Event to base calculations on:

Event to base calculations on:

Select the event that will correspond to the date you will be selecting. The entry form will update to accommodate your selection.

Due date:


Select the month, day, and year that corresponds to the selected date type.

Ultrasound weeks & days:Ultrasound weeks & days:Ultrasound weeks & days along:Ultrasound weeks and days along:

Ultrasound weeks and days along:

If you selected "Ultrasound date" as the event, select the number of weeks and days along you are according to your ultrasound. If you selected a different date event, the fields on this row will be disabled.

Average cycle:

Average menstrual cycle:

If you selected "1st day of last period" date to base the calculation on, select the number of days your average menstrual cycle lasts. If you selected a different date type, then the field on this row will be disabled.

Date:Conceive date:Conception date:Estimated date of conception:

Conception date:

Based on your entries, this is the estimated conception date.

If the calculation is based on Due or birth date, the result is 266 days subtracted from the selected due or birth date (266 is the average gestation period per Naegele's Rule).

If the calculation is based on the 1st day of last period, the result is equal to the 1st day of your last period plus 14 days, plus your average menstrual cycle minus 28 days (30 days - 28 days = 2 + 14 = 16 days).

If the calculation is based on the ultrasound weeks and days along, the result is equal to the ultrasound date minus the total days along (1st day of last period), plus 14 days (14 days after 1st day of last period).

Period:Last period:Last period date:Estimated last period date:

Last period date:

This is typically 14 days prior to the conception date (assuming ovulation occurred on day 14 of your menstrual cycle), though this can vary if a woman's average menstrual cycle is different from the 28-day norm.

Due date:Due date:Due date:Due date:

Due date:

This is 266 days from your conception date.

If you would like to save the current entries to the secure online database, tap or click on the Data tab, select "New Data Record", give the data record a name, then tap or click the Save button. To save changes to previously saved entries, simply tap the Save button. Please select and "Clear" any data records you no longer need.

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Get the App!

$0.99, iPhone or Android.

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QR Code for Conception Date Calculator App
Help and Tools

Help and Tools:

This section, which you can Unstick or Hide in the Calculator Preferences (just above the top of the calculator), contains the following tabs (content too long to fit within bordered frame can be scrolled up and down):

About: Click this tab for an introduction to the calculator.

Instructions: Click this tab for step-by-step instructions for using the calculator.

Terms: Click this tab for a list of the descriptions that are located within each popup help button (info icons).

PCalc: Click this tab for a handy "pocket" calculator you can use when you need to calculate an entry needed for the calculator.

Data: Click this tab to save a set of entries or a note in between visits. Entries and notes will be stored to your web browser's local storage (if supported by your browser of choice), meaning they can only be recalled with the same device and web browser you were using when you saved them. If you'd like to save entries and notes between devices you can do so by subscribing to the Ad-Free Member Version.



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Adjust Calculator Width:

Move the slider to left and right to adjust the calculator width. Note that the Help and Tools panel will be hidden when the calculator is too wide to fit both on the screen. Moving the slider to the left will bring the instructions and tools panel back into view.

Also note that some calculators will reformat to accommodate the screen size as you make the calculator wider or narrower. If the calculator is narrow, columns of entry rows will be converted to a vertical entry form, whereas a wider calculator will display columns of entry rows, and the entry fields will be smaller in size ... since they will not need to be "thumb friendly".

Show/Hide Popup Keypads:

Select Show or Hide to show or hide the popup keypad icons located next to numeric entry fields. These are generally only needed for mobile devices that don't have decimal points in their numeric keypads. So if you are on a desktop, you may find the calculator to be more user-friendly and less cluttered without them.

Stick/Unstick Tools:

Select Stick or Unstick to stick or unstick the help and tools panel. Selecting "Stick" will keep the panel in view while scrolling the calculator vertically. If you find that annoying, select "Unstick" to keep the panel in a stationary position.

If the tools panel becomes "Unstuck" on its own, try clicking "Unstick" and then "Stick" to re-stick the panel.