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Pregnancy Calculator

Calculate your due date, current gestational age, and pregnancy milestones from your last menstrual period, conception date, IVF transfer, or known due date. Buddhist Era dates supported in Thailand.

MM/DD/YYYY

days

Average length of your menstrual cycle. Default is 28 days.

Choose how you want to calculate the due date.
EDD is unchanged; the recommended delivery week is typically earlier.
Calculate at a different date
MM/DD/YYYY

Enter a date to see your due date and pregnancy week.

Also available:

Ovulation Calculator
This calculator is for informational purposes only and does not replace professional medical advice. Please consult your obstetrician for clinical decisions.All calculations happen in your browser. We do not store, transmit, or share any of the dates you enter.
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Pregnancy calculator. Due date, gestational age, trimester, and ACOG milestone timeline.

A pregnancy calculator finds your due date, gestational age, and trimester from your last period date using Naegele's 280-day rule. It also accepts IVF transfer dates, conception dates, and known due dates as alternative anchors.

What Is a Pregnancy Calculator?

A pregnancy calculator is a tool that takes a dating anchor — your last menstrual period (LMP), conception date, IVF transfer date, or known due date — and computes your estimated due date (EDD), current gestational age, trimester, and how far along you are as a percentage of the full 280-day pregnancy.
The calculator uses Naegele's rule as its primary formula: EDD = LMP + 280 days. This rule, endorsed by ACOG Committee Opinion 700, assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the rule is adjusted by adding or subtracting the difference: EDD = LMP + 280 + (cycle length − 28). For a 30-day cycle, for example, the EDD moves two days later.
Pregnancy is divided into three trimesters. The first trimester spans weeks 1 through 13 (LMP day 1 to day 97). The second trimester runs from week 14 through week 27 (day 98 to day 195). The third trimester begins at week 28 (day 196) and ends at delivery. ACOG Committee Opinion 579 further refines the final weeks into early term (37w0d–38w6d), full term (39w0d–40w6d), late term (41w0d–41w6d), and post-term (42w0d onward).
Gestationally, you are considered pregnant from day 1 of your last period, not from the day of conception. This means a woman who is '6 weeks pregnant' by her OB's calculation conceived approximately 4 weeks ago — a common source of confusion for first-time parents.
This calculator is 100% client-side. Nothing is sent to a server, no account is required, and your dates stay on your device.

How to Use the Pregnancy Calculator

To get your due date and gestational age, follow these steps:
1. Enter the first day of your last menstrual period (LMP). This is the date your most recent period began — not the day it ended. If you are unsure of the exact date, use the closest Monday of the week it started.
2. Set your cycle length. The default is 28 days, which works for most women. If your cycles are typically shorter or longer, adjust this field. The valid range is 21 to 35 days. This allows the calculator to shift the EDD by the correct number of days.
3. Read your results. The calculator immediately shows your estimated due date, your gestational age as of today in weeks and days, your current trimester, and what percentage of the pregnancy is complete.
4. For alternative anchors, open the 'More options' section. From there you can switch to: Conception date (the calculator adds 266 days to derive the EDD); IVF embryo transfer (enter the transfer date and select Day 3, Day 5, or Day 6 — the calculator computes EDD = transfer date + (266 − embryo age)); or a known due date (the calculator reverse-derives your LMP equivalent and displays your gestational age as of today).
5. Open 'View milestone timeline' to see every ACOG-aligned prenatal appointment and screening window anchored to your actual dates — NT scan, anatomy scan, glucose tolerance test, Tdap vaccination window, GBS test, and the full-term / post-term thresholds.
6. Open 'Week-by-week development' to see your baby's approximate length, weight, and developmental stage at each gestational week from week 4 to week 42, with your current week highlighted.

Pregnancy Due Date Formulas

EDDLMP=LMP+280+(C−28)EDD_{\text{LMP}} = LMP + 280 + (C - 28)
  • EDDEDD = Estimated due date
  • LMPLMP = First day of last menstrual period
  • CC = Cycle length in days (default 28)
  • GAGA = Gestational age in days = today − LMP-equivalent
  • dd = Embryo age at transfer in days (3, 5, or 6)
The base formula is Naegele's rule, which counts 280 days (40 weeks) from the LMP:
EDD=LMP+280+(C−28)EDD = LMP + 280 + (C - 28)
For a standard 28-day cycle this simplifies to $EDD = LMP + 280$. Gestational age in days is then:
GAdays=today−LMPGA_{days} = \text{today} - LMP
For IVF embryo transfers, the fertilization date is known precisely, so the calculation starts from fertilization:
EDDIVF=transfer date+(266−d)EDD_{\text{IVF}} = \text{transfer date} + (266 - d)
For example, a Day 5 blastocyst transfer: $266 - 5 = 261$, so the EDD is 261 days after the transfer date. A Day 3 transfer: $266 - 3 = 263$ days after transfer. A Day 6 transfer: $266 - 6 = 260$ days.
For a known conception date the formula is:
EDDconception=conception date+266EDD_{\text{conception}} = \text{conception date} + 266
All four anchors converge on the same canonical LMP-equivalent, which drives every downstream output — gestational age, trimester classification, and the milestone timeline.
AnchorDays addedNotes
LMP (28-day cycle)+280Standard Naegele's rule
LMP (non-28-day cycle)+280 ± (C − 28)Cycle-adjusted
Conception date+266Fertilization to birth
IVF Day 3 transfer+263266 − 3
IVF Day 5 transfer+261266 − 5
IVF Day 6 transfer+260266 − 6
Known EDD (reverse)−280Derives LMP-equivalent

Pregnancy Calculator Examples

Example 1 — Standard LMP calculation (28-day cycle)

Sarah's last period started on March 10, 2025. Her cycle is 28 days. The calculator adds 280 days: EDD = October 14, 2025. As of May 1, 2025, she is 51 days into the pregnancy — 7 weeks and 2 days — firmly in the first trimester (weeks 1–13). She is 18% of the way through her pregnancy. The milestone timeline tells her that her NT scan window opens at LMP + 77 days (May 26, 2025), and her anatomy scan will be scheduled between LMP + 126 days (July 14) and LMP + 154 days (August 11).

Example 2 — IVF Day 5 blastocyst transfer

Maria had a Day 5 frozen blastocyst transfer on February 14, 2025. The formula applies 266 − 5 = 261 days: EDD = November 2, 2025. Her LMP-equivalent is February 9, 2025 (transfer date minus 14 + 5 days = 19 days back), giving her a gestational age of 11w 4d on May 1, 2025. She is in the first trimester and her NT scan window is just opening. Because the conception date is precisely known, IVF due dates are generally more accurate than LMP-based estimates.

Example 3 — Reverse calculation from a known due date

James and Lisa already know their due date from their dating ultrasound: December 25, 2025. Entering this into the 'Known due date' mode, the calculator derives the LMP-equivalent as March 19, 2025 (December 25 minus 280 days). As of May 1, 2025, gestational age is 6 weeks and 3 days — early first trimester. The milestone timeline shows the dating ultrasound window opened at 8w (May 13, 2025), so their next appointment is imminent. This mode is useful for anyone whose OB has already provided a due date and who wants the full milestone calendar anchored to that date.

Example 4 — Irregular cycle (33-day cycle)

Emma has a 33-day cycle. Her LMP started on January 15, 2025. With a standard 28-day assumption she would get an EDD of October 22, 2025. With cycle length set to 33 days, the calculator adds 5 extra days: EDD = October 27, 2025. Missing this adjustment would make her appear 5 days behind schedule on every gestational-age check — a meaningful discrepancy during first-trimester screening. Women with consistently irregular cycles are advised by ACOG to confirm dating with an early ultrasound (weeks 8–14) regardless.

What to Do After Getting Your Due Date

  • Schedule your first prenatal visit as soon as you see a positive test. Most OBs and midwives book the initial appointment between 8 and 10 weeks. The earlier you go, the sooner blood type, rubella immunity, and other baseline labs are drawn, and the more time your provider has to address any risk factors.
  • Use the milestone timeline to put key appointments in your calendar now. The NT scan (11–13 weeks), anatomy scan (18–22 weeks), glucose tolerance test (24–28 weeks), and GBS test (36–37 weeks) all have narrow windows. Missing them by even a few weeks can mean rescheduling or losing the screening's value.
  • Treat your due date as the midpoint of a two-week window, not a deadline. Only about 4–5% of babies are born on the exact EDD. Approximately 90% of full-term births happen between 38 and 42 weeks. Plan your leave and hospital bag preparation for week 37 onward, not the EDD itself.
  • If your cycle is consistently different from 28 days, adjust the cycle length field before trusting the dates. A woman with a 35-day cycle who leaves it at 28 days will see dates that are systematically 7 days off — which can create needless anxiety about being 'behind' at every scan.
  • For twins or higher-order multiples, the EDD computed here uses the same Naegele formula as for singletons — it does not change. What changes is the recommended delivery week: 37w for di-chorionic/di-amniotic twins, 36w for mono-chorionic/di-amniotic, 32–34w for mono-chorionic/mono-amniotic. Use the milestone calendar to find these thresholds and discuss them with your specialist.
  • Save or bookmark your result URL. The calculator stores all your inputs in the URL so you can share the exact same dates with a partner, parent, or doctor without re-entering anything.
  • Remember that a dating ultrasound performed between 8w and 13w 6d is the gold standard for pregnancy dating per ACOG. If the ultrasound date differs from your calculated date by more than 7 days, your provider will typically redate to the ultrasound measurement. The calculator gives an excellent first estimate, but early ultrasound should always confirm it.

Frequently Asked Questions About the Pregnancy Calculator

When is my baby due if my last period started on a specific date?

Add 280 days to the first day of your last menstrual period. For a 28-day cycle, this is exactly 40 weeks. If your period started on January 1, your due date is October 8. The calculator above does this instantly and adjusts for cycle lengths other than 28 days.

How many weeks pregnant am I?

Subtract the first day of your last period from today's date, then divide by 7. If your LMP was 10 weeks ago, you are 10 weeks pregnant by standard gestational counting. The calculator shows this as weeks and days (e.g., '10w 3d'). Remember: gestational age starts from the LMP, not from conception, so you are technically 'pregnant' for the first two weeks before fertilization occurs.

How do I calculate my due date from my conception date?

Add 266 days (38 weeks) to your conception or ovulation date. If you know you ovulated on January 14, your EDD is October 7. Use the 'Conception date' anchor in the 'More options' section of the calculator for an instant result.

How is an IVF due date calculated for a Day 5 embryo transfer?

For a Day 5 blastocyst transfer, the EDD equals the transfer date plus 261 days (266 minus 5). For a Day 3 transfer, add 263 days. For a Day 6 transfer, add 260 days. Select 'IVF transfer' in the 'More options' section, enter your transfer date, and choose the embryo age. The calculator handles the math automatically.

What is gestational age and how is it different from fetal age?

Gestational age counts from the first day of the last menstrual period and is the standard clinical measure. Fetal (conceptional) age counts from actual fertilization and is typically 2 weeks less. If your OB says you are '12 weeks pregnant', your baby's fetal age is approximately 10 weeks. All due date calculators and prenatal milestones use gestational age.

Is this pregnancy calculator accurate?

Yes. It uses ACOG-endorsed Naegele's rule with cycle-length correction, and the IVF formula matches published ASRM guidance. However, no calculator can be more accurate than a first-trimester dating ultrasound (margin of error ±5–7 days). Only 4–5% of babies are born on the exact EDD. Use this tool as a reliable starting point and confirm with your provider at the 8–12 week visit.

Do I need to create an account or sign up?

No. This calculator is completely free and requires no account, email address, or personal information. All calculations run in your browser — nothing is stored or sent to a server. Share your results via the URL, which encodes all your inputs.

What are the trimester boundaries in weeks?

The first trimester is weeks 1 through 13 (days 1–97 from LMP). The second trimester is weeks 14 through 27 (days 98–195). The third trimester begins at week 28 (day 196) and ends at delivery. ACOG also classifies the final weeks as early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42 weeks and beyond).

Can I use this calculator for twins or multiples?

Yes, with an important note: the EDD calculated here is the same as for a singleton and is used as the reference date. However, the recommended delivery week differs significantly — typically 37 weeks for di-chorionic twins versus 39–40 weeks for singletons. Enable the 'Twins or more' toggle in 'More options' to see a note about this in your results, and always follow your specialist's guidance for multifetal pregnancies.

When should I schedule my first prenatal visit?

Most providers schedule the first prenatal visit between 8 and 10 weeks of pregnancy. Calling as soon as you have a positive test (even if you are only 4–5 weeks along) is advisable, since popular practices can book weeks in advance. At this visit, your provider will confirm the pregnancy, draw baseline labs, and order your dating ultrasound.

What is Naegele's rule?

Naegele's rule calculates the estimated due date by adding 280 days — or 9 calendar months plus 7 days — to the first day of the last menstrual period. Named after German obstetrician Franz Karl Naegele (1778–1851), it remains the standard formula endorsed by ACOG, the NHS, and virtually every maternity care system worldwide.

What does the milestone timeline show?

The milestone timeline converts the standard ACOG prenatal schedule into actual calendar dates anchored to your specific pregnancy. It shows the NT scan window (11–13 weeks), dating ultrasound (8–13 weeks), anatomy scan (18–22 weeks), glucose tolerance test (24–28 weeks), Tdap vaccine window (27–36 weeks), GBS test (36–37 weeks), and the early-term, full-term, late-term, and post-term thresholds — all as real dates calculated from your due date.

Pregnancy Terms Explained

Estimated Due Date (EDD)

The date 280 days (40 weeks) after the first day of the last menstrual period. It is a statistical midpoint, not a guaranteed delivery date — about 4–5% of babies are born exactly on this date, and 90% deliver between 38 and 42 weeks.

Gestational Age (GA)

The age of the pregnancy measured in weeks and days from the first day of the last menstrual period. Standard clinical measure used for all prenatal scheduling. Different from fetal age, which starts from conception and is typically 2 weeks less.

Naegele's Rule

The formula for calculating EDD: add 280 days to the LMP (or equivalently, subtract 3 months and add 7 days to the LMP). Endorsed by ACOG Committee Opinion 700 as the standard method. Assumes a 28-day cycle; adjusted for other cycle lengths.

Last Menstrual Period (LMP)

The first day of your most recent menstrual period. The primary anchor for pregnancy dating. Because ovulation and fertilization typically occur 14 days after the LMP, the LMP predates conception by approximately two weeks.

Trimester

One of three roughly equal periods dividing pregnancy. First trimester: weeks 1–13 (LMP to day 97). Second trimester: weeks 14–27 (day 98–195). Third trimester: weeks 28–42+ (day 196 to delivery). ACOG further divides the final weeks into early term, full term, late term, and post-term.

IVF Transfer

In in-vitro fertilization, the procedure where a fertilized embryo is placed into the uterus. The embryo's age at transfer (Day 3, Day 5, or Day 6) is subtracted from 266 to determine the number of days to add to the transfer date to get the EDD.

NT Scan (Nuchal Translucency)

An ultrasound scan performed between 11 weeks 0 days and 13 weeks 6 days that measures fluid at the back of the baby's neck to screen for chromosomal conditions including Down syndrome.

Anatomy Scan

A detailed ultrasound performed between 18 and 22 weeks of pregnancy that checks fetal anatomy including brain, heart, kidneys, limbs, and placenta position. Also called the morphology scan or mid-pregnancy scan.

GBS Test (Group B Streptococcus)

A vaginal and rectal swab performed between 36 and 37 weeks to detect Group B Streptococcus bacteria. If positive, intravenous antibiotics are given during labor to protect the newborn from infection.

Periviable

The gestational age window between 22 weeks 0 days and 23 weeks 6 days (ACOG/SMFM Obstetric Care Consensus #6) where intensive care may provide a chance of survival. Survival rates increase sharply from this point to viability at 24 weeks.

Content verified by the Smart Calculators Team