Reverse Due Date vs Gestational Age
The reverse due date calculator and the gestational age calculator answer the same pregnancy-timing question from opposite directions. The gestational age calculator counts forward from a dating basis — last menstrual period, conception date, or an ultrasound — to the gestational age in weeks and days as of a chosen date. The reverse due date calculator works backward from a due date to estimate the last menstrual period, likely conception timing, trimester boundaries, milestones, and appointment windows. They share the same standard dating conventions: about 280 days for a pregnancy counted from the first day of the last menstrual period, with conception estimated about 14 days later in a typical cycle.
Both tools are planning and education aids, not medical dating. Irregular cycles, assisted reproduction dates, ultrasound findings, and clinician judgment can all produce different timelines, so confirm dates and care timing with a healthcare professional.
What each calculator does
The reverse due date calculator takes one due date. It subtracts 280 days for the estimated last menstrual period and treats that as gestational week 0, then adds 14 days for the estimated conception date. It ends the first trimester at week 13, ends the second trimester at week 26, and starts the third trimester at week 27, and it lists development milestones from week 4 through week 40 and appointment prompts from week 8 through week 38. For example, a due date of October 8, 2026 returns January 1, 2026 as the estimated LMP and January 15, 2026 as the estimated conception date. It is the right tool when the date you know is the due date, not the period date that started the pregnancy clock.
The gestational age calculator has three calculation paths. In the last menstrual period path, it counts calendar days from the LMP to the calculate-as-of date. In the conception path, it subtracts 14 days from the conception date to create a dating start. In the ultrasound path, it converts the scan’s weeks and days to days and adds the elapsed time since the scan. The result is expressed as completed weeks and days, with the due date set 280 days after the dating start and the estimated conception date 14 days after it. It rejects results above 42 weeks and requires the dating input to be on or before the as-of date. Use it when you want a forward count in weeks and days from a known dating basis.
Side-by-side comparison
| Feature | Reverse Due Date Calculator | Gestational Age Calculator |
|---|---|---|
| Direction | Backward from a due date | Forward from a dating basis |
| Starting input | One due date | LMP date, conception date, or ultrasound scan date |
| Primary output | Calendar dates: estimated LMP, conception, trimester ends, milestones, appointments | Completed weeks and days as of a chosen date |
| Core conventions | Due date − 280 days = LMP; +14 days = conception | Dating start → count days to as-of; due date = start + 280 days |
| Trimester boundaries | End of first at week 13, end of second at week 26, third from week 27 | Same week-based boundaries |
| Domain limits | None on input; estimates only | Rejects above 42 weeks; dating input must precede as-of date |
When to use which
Use the reverse due date calculator when the anchor you have is the due date — planning backward for a target delivery month, reconciling a clinician’s due date with an estimated LMP and conception window, or placing milestones and appointment prompts on a calendar before pregnancy. Use the gestational age calculator when you have a dating basis and a date you care about — today, or a past appointment date you want to reproduce — and want the answer in completed weeks and days, along with the due date and trimester. If you are planning before conception, the reverse view is a rough calendar tool: the estimated conception date is a midpoint implied by the due date, not proof of a specific day, and real fertility timing varies. Neither calculator should be used for decisions about care, testing, medication, travel, or delivery planning, or to settle questions about conception timing or paternity.
Where to start
- Reverse due date calculator — estimate LMP, conception, trimester boundaries, milestones, and appointment windows from a due date.
- Gestational age calculator — count gestational age in weeks and days from LMP, conception, or ultrasound dating.
Informational note: This page is an educational comparison. Pregnancy dating affects screening windows and care timing, and a calculator cannot decide which clinical date should be authoritative. Irregular cycles, assisted reproduction dates, and ultrasound findings can change the timeline, so confirm dates and prenatal care with a qualified healthcare professional.