Pregnancy Date Calculator

Two pink lines change everything — and the first question most people ask is "when is the baby due?" The Pregnancy Date Calculator answers that and more in seconds: enter the first day of your last period, your average cycle length, and the date you want to calculate from, and it instantly estimates your due date, your likely conception date, and how many weeks pregnant you are. Whether you just got the news or you are planning ahead, this free tool translates one date you know into the three dates that organize the entire pregnancy.

These three dates are the backbone of prenatal care. The due date schedules every ultrasound, screening, and appointment. The conception date anchors the timeline of early development. And the current week determines which milestones apply right now — when the heartbeat becomes detectable, when screening windows open, when viability approaches. The Pregnancy Date Calculator puts all three in front of you instantly, so you can walk into your first prenatal visit already oriented.

What Is the Pregnancy Date Calculator?

The Pregnancy Date Calculator is a free pregnancy-dating tool that estimates key dates from your menstrual history. Its three inputs are the first day of your last menstrual period (LMP), your average cycle length in days, and the date you want to calculate from (usually today). Its three outputs are the estimated due date (EDD), the estimated conception date, and the number of completed weeks of pregnancy as of your chosen date.

The method is the standard Naegele's rule used in obstetrics for generations, adjusted for your actual cycle length: due date = LMP + 280 days + (cycle length − 28). The 280 days (40 weeks) is the average human gestation measured from the last period — which, note, means the "first two weeks of pregnancy" occur before conception, a quirk of medical dating that confuses nearly everyone at first. Conception is estimated as LMP + (cycle length − 14), placing it at ovulation, and completed weeks are simply the days elapsed since LMP divided by seven.

This is the same dating logic behind the due date your provider will initially assign. Later, a first-trimester ultrasound may refine it — ultrasound dating in early pregnancy is accurate to about five to seven days and takes precedence if it differs meaningfully. The calculator gives you the LMP-based estimate that starts every pregnancy conversation.

Why Pregnancy Is Dated From the Last Period

Dating from the last period instead of conception seems backwards until you consider the practical problem: most people know when their period started, but almost nobody knows the exact moment of conception. The LMP is an observable, recordable event; conception is invisible. So medicine built the entire system — all 40 weeks — on the date women can reliably report.

This creates the famous two-week offset: at "four weeks pregnant," the embryo has existed for only about two weeks. Gestational age (from LMP) and embryonic or fetal age (from conception) differ by roughly fourteen days in a 28-day cycle. Every milestone you read about — "week 6: heartbeat detectable" — uses gestational age, the LMP-based count. The calculator's weeks-pregnant output follows this convention, so its number will match what your provider and every pregnancy app tells you.

The cycle-length adjustment matters because ovulation — and therefore conception — shifts with cycle length. In a 35-day cycle, conception happens around day 21, a week later than the 28-day textbook. Without adjustment, the due date would be a week early. The calculator's +(cycle − 28) correction keeps the estimate honest for longer and shorter cycles alike.

How to Use the Pregnancy Date Calculator

One date you know, two quick settings:

1. Enter the first day of your last period. Select the date bleeding began using the date picker. Accuracy here drives everything — check a calendar or tracking app if unsure.
2. Enter your average cycle length. First day of one period to the first day of the next, averaged over recent cycles. The default is 28; adjust to your real number.
3. Enter the date to calculate from. This defaults to today — leave it to see your current week, or pick a future date to see how far along you will be then.
4. Click Calculate. Read your estimated due date, estimated conception date, and completed weeks of pregnancy.
5. Bring the results to your provider. Your LMP-based due date is exactly what the first prenatal visit starts with; the ultrasound will confirm or adjust it.

Tip: to see which week you will be on a specific future date — a vacation, a work deadline, a family event — just change the "calculate from" date and re-run. It is a handy way to plan around the pregnancy timeline.

Worked Example 1: LMP January 5, 28-Day Cycle

Suppose Sofia's last period began on January 5, 2026, her cycles are 28 days, and she wants to know her dates as of March 16, 2026. She enters 2026-01-05, 28, and 2026-03-16, then clicks Calculate.

Due date: January 5 + 280 days + (28 − 28) = January 5 + 280 days = October 12, 2026. (Naegele's shortcut: subtract 3 months, add 7 days — January 5 becomes October 12.) Conception date: January 5 + (28 − 14) = January 19, 2026. Weeks pregnant: January 5 to March 16 is 70 days; 70 ÷ 7 = 10 complete weeks.

Outputs: Estimated Due Date October 12, 2026; Estimated Conception Date January 19, 2026; Weeks Pregnant: 10 weeks. At 10 weeks Sofia is nearing the end of the first trimester — the dating ultrasound window, when measurements are most accurate, is right now. The calculator has effectively told her not just her dates but her next medical step.

Worked Example 2: LMP February 10, 34-Day Cycle

Now consider Hannah, whose last period began February 10, 2026, with longer 34-day cycles, calculating as of April 1, 2026. She enters 2026-02-10, 34, and 2026-04-01.

Due date: February 10 + 280 days + (34 − 28) = February 10 + 286 days = November 23, 2026. Without the cycle adjustment, the estimate would have been November 17 — six days early, enough to mislabel a full-term baby as overdue. Conception date: February 10 + (34 − 14) = March 2, 2026. Weeks pregnant: February 10 to April 1 is 50 days; 50 ÷ 7 = 7 complete weeks (7 weeks, 1 day).

Outputs: Estimated Due Date November 23, 2026; Estimated Conception Date March 2, 2026; Weeks Pregnant: 7 weeks. Hannah's example shows why the cycle-length input earns its place: six days of correction at the due date, and a conception date three weeks after the LMP rather than two — details that change screening schedules and "how far along" conversations with her provider.

Trimesters and Key Milestones by Week

With your weeks-pregnant number in hand, the pregnancy map unfolds. The first trimester (weeks 1–13) covers conception, implantation, organ formation, and the end of the highest miscarriage-risk period; the dating ultrasound and first-trimester screening happen here. The second trimester (weeks 14–27) brings the anatomy scan around week 20, quickening (first felt movement), and glucose screening. The third trimester (weeks 28–40) covers rapid growth, Group B strep testing, and birth planning, with "full term" at 39–40 weeks.

A few landmarks worth knowing: the embryonic heartbeat is typically detectable by transvaginal ultrasound around week 6; the nuchal translucency screening window is weeks 11–14; viability — the point where intensive care can sustain a premature baby — is around 23–24 weeks; and only about 5% of babies arrive on the due date itself, with most coming within two weeks on either side. The calculator's due date is the center of a range, not an appointment.

Ultrasound Dating: When the Due Date Gets Refined

The LMP-based estimate assumes ovulation occurred on schedule — but cycles vary, and memory of the LMP is not always exact. That is why the first-trimester ultrasound (ideally weeks 8–13) measures the embryo's crown-rump length to date the pregnancy directly, accurate to about five to seven days. If the ultrasound date differs from the LMP date by more than that margin, clinical guidelines say to adopt the ultrasound date.

This does not mean the calculator was "wrong" — it means dating is a two-step process, and the calculator performs step one faithfully. After the dating scan, use the ultrasound-confirmed due date for everything going forward, and you can even back-calculate an adjusted LMP from it. Later ultrasounds are far less accurate for dating (growth varies between babies), so the first-trimester measurement is the one that counts.

Special cases deserve mention: with irregular cycles, the LMP estimate is shaky from the start — flag this for your provider and expect ultrasound to take over dating. With IVF, dating is exact from the transfer date (fetal age plus two weeks), no calculator needed. And with recent hormonal contraception or breastfeeding, the first post-pill bleed may not reflect true ovulation timing.

What If You Do Not Know Your Last Period Date?

Not everyone can answer the calculator's first question. If your periods are irregular, if you conceived while breastfeeding without a resumed cycle, if you came off hormonal contraception recently, or if you simply were not tracking, the LMP may be a guess — and a guessed LMP produces a guessed due date. The honest approach is to enter your best estimate, note the uncertainty, and let the first-trimester ultrasound take over dating duties. Be upfront with your provider about the uncertainty rather than presenting a shaky date as fact; the dating scan exists precisely for this situation, and clinicians would much rather date from a clean ultrasound than from a confident-sounding guess.

A few specific scenarios deserve mention. After stopping birth control pills, the first withdrawal bleed is not a true menstrual period — ovulation timing for the first genuine cycle is unpredictable, so LMP-based dating from that bleed can be off by weeks. While breastfeeding, ovulation can return before the first postpartum period, meaning conception can occur with no LMP at all; here the calculator cannot help and ultrasound dating is the entire plan. With very irregular cycles, some providers skip LMP dating altogether and schedule an early scan as the primary dating method. In every one of these cases the calculator still has a role — it gives you a provisional timeline to plan around right now — but treat its outputs as placeholders stamped "pending ultrasound confirmation" until the scan weighs in.

Tips for Using Your Pregnancy Dates Well

1. Record your LMP accurately. Everything flows from this date. Note the first day of bleeding (not spotting) in a calendar or app every cycle — future you will be grateful.
2. Know your real cycle length. Track several cycles and average them. A wrong cycle length shifts the due date by the same number of days.
3. Treat the due date as a range. Plan for the baby arriving any time from two weeks before to two weeks after the estimated date. Only 1 in 20 arrives on the day itself.
4. Schedule the dating ultrasound promptly. The 8–13 week window gives the most accurate confirmation. The calculator tells you when that window is — use it.
5. Use the week count to time screenings. Many tests have narrow eligibility windows (nuchal translucency at 11–14 weeks, anatomy scan near 20). Your weeks-pregnant figure keeps you on schedule.
6. Do not panic over a "late" due date. First babies especially often arrive past the estimated date. Providers monitor post-term pregnancies closely; the date guides surveillance, not alarm.
7. Share dates with your support circle carefully. Many parents share only the month publicly to avoid a barrage of "is the baby here yet?" messages in the final weeks.
8. Start folic acid and prenatal care early. The neural tube forms in weeks 4–6 — often before you know you are pregnant. If planning, begin folic acid and book the first visit as soon as pregnancy is confirmed.

Frequently Asked Questions

1. What does the Pregnancy Date Calculator do?

It estimates your due date, conception date, and current weeks of pregnancy from the first day of your last period, your cycle length, and your chosen calculation date.

2. Is the Pregnancy Date Calculator free?

Yes — free, instant, no sign-up, and your dates stay private in your browser.

3. Why is pregnancy dated from the last period instead of conception?

Because the last period is an observable date most people know, while conception is invisible. All 40 weeks of obstetric dating are built on this convention.

4. Does that mean I am "two weeks pregnant" before conception?

In gestational-age terms, yes — the first two weeks of the 40-week count precede ovulation in a 28-day cycle. It is a dating convention, not a biological claim.

5. How is the due date calculated?

With Naegele's rule adjusted for cycle length: LMP + 280 days + (cycle length − 28). For 28-day cycles this simplifies to LMP + 9 months + 7 days.

6. How accurate is the estimated due date?

It is a solid estimate for regular cycles, but only about 5% of babies arrive on the due date. First-trimester ultrasound refines it to within about a week.

7. What if my cycles are irregular?

The LMP-based estimate is less reliable. Tell your provider, who will rely more heavily on early ultrasound dating to establish your due date.

8. What if I do not remember my last period date?

Give your best estimate and let the dating ultrasound correct it. Going forward, tracking cycles in an app prevents this problem.

9. Can the calculator be used for IVF pregnancies?

IVF dating is exact and does not need estimation: gestational age is the embryo's age plus two weeks (for a day-5 transfer, add 19 days to the transfer date).

10. What does "7 weeks pregnant" actually mean for development?

It means about 5 weeks since conception. At 7 gestational weeks the embryo has a detectable heartbeat and early limb buds — milestones your provider tracks by this same count.

11. Will my due date change at the ultrasound?

Possibly. If first-trimester ultrasound dating differs from the LMP date beyond the expected margin, guidelines recommend adopting the ultrasound date.

12. What are the trimesters?

First: weeks 1–13. Second: weeks 14–27. Third: weeks 28–birth. Each has characteristic milestones and screenings your provider will schedule by week.

13. Is it normal to go past the due date?

Yes — most pregnancies continue past the estimated date, especially first ones. Providers typically monitor closely and discuss options around 41–42 weeks.

14. Can I use a future date in the calculator?

Yes. Enter any future "calculate from" date to see how many weeks pregnant you will be then — useful for planning travel, work, and events.

15. Is my information kept private?

Entirely. All calculations run locally in your browser; nothing is transmitted, stored, or shared.

CONCLUSION

The Pregnancy Date Calculator turns the single date every pregnancy starts with — the first day of your last period — into the three dates that structure everything after: your due date, your conception date, and your current week. Use it the moment you learn the news, use it to plan around milestones and screenings, and bring its results to your first prenatal visit where ultrasound will refine them. Forty weeks is a long journey; knowing exactly where you stand on the map is the best possible start.