🤰 Due Date Calculator — Find Your Pregnancy Due Date
Enter your last menstrual period, conception date, or IVF transfer date to get your estimated due date, trimester timeline, and key milestone weeks.
TRIMESTER TIMELINE
KEY MILESTONE WEEKS
Pregnancy Quick Facts
Sources: ACOG, WHO, NICE guidelines
How Your Due Date Is Calculated: Naegele's Rule Explained
The most widely used method for calculating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in 1812. The formula adds 280 days (40 weeks) to the first day of your last menstrual period.
If your cycle is longer than 28 days, ovulation happens later, so the due date shifts forward. A 35-day cycle adds 7 days; a 21-day cycle subtracts 7 days.
When calculating from a conception date, 266 days (38 weeks) are added, since fertilisation typically occurs 14 days after the LMP — 280 − 14 = 266.
Three ways to calculate your due date
Trimester Timeline & Key Development Milestones
A week-by-week guide to the major stages of pregnancy and what to expect at each.
| Trimester | Weeks | Key Events & Milestones | What to Expect |
|---|---|---|---|
| First Trimester | Weeks 1–13 | Implantation (wk 3), heartbeat (wk 6), all major organs forming, nuchal scan (wk 11–14) | Nausea, fatigue, breast tenderness, frequent urination. Risk of miscarriage highest. |
| Week 20 | Anatomy Scan | Detailed ultrasound to check baby's anatomy, sex can often be determined | Baby is now about 25 cm. Movement (quickening) felt around this time. |
| Week 24 | Viability | Baby reaches threshold of survival outside the womb with intensive medical care | Lung development continues. Baby responds to sounds. |
| Second Trimester | Weeks 14–26 | Baby grows rapidly, hair and fingerprints form, movement felt | Nausea usually subsides. Energy returns. Baby bump visible. |
| Week 28 | Third Trimester | Baby's eyes open, brain development accelerates, immune system matures | Braxton Hicks contractions may start. Glucose screening test. |
| Week 37 | Full Term | Baby is considered full term. Lungs fully mature, ready for birth | Baby may engage (drop) into the pelvis. Nesting instinct common. |
| Third Trimester | Weeks 27–40 | Final weight gain, lung maturation, baby turns head-down | Heartburn, back pain, swelling. Birth plan preparation. |
| Week 40 | Due Date | Estimated due date based on Naegele's rule | Only 5% of babies arrive on this exact day. Labour signs: contractions, water breaking. |
Source: ACOG, NHS, American Pregnancy Association.
Frequently Asked Questions
Due Date Calculator by Last Period — How Accurate Is It?
The LMP (last menstrual period) method is the most common way to calculate a due date, and it works well for women with regular 28-day cycles. The formula, known as Naegele's rule, simply adds 280 days to the start of your last period.
The accuracy depends heavily on cycle regularity. A woman with a consistent 28-day cycle will get a very reliable estimate. However, if your cycles vary by more than a few days month to month, the LMP estimate can be off by a week or more. This is why cycle length adjustment matters — our calculator lets you input your actual average cycle length (21–35 days) to refine the estimate.
Early ultrasound (6–12 weeks) is the most accurate dating method. A crown-rump length (CRL) measurement at the first trimester scan can pinpoint gestational age to within 5–7 days. If the ultrasound date differs from your LMP date by more than 7 days (in the first trimester), clinicians typically revise the due date to the ultrasound estimate.
Factors that reduce LMP accuracy include: irregular periods, recent hormonal contraception use, breastfeeding cycles, postpartum return to fertility, and polycystic ovary syndrome (PCOS). In these situations, a conception-based calculation or early ultrasound dating is recommended.
IVF Due Date Calculator — Day 3 vs Day 5 Transfer
For pregnancies achieved through IVF (in vitro fertilisation), the due date is calculated differently because the exact timing of fertilisation and embryo development is known. The two most common transfer types are Day 3 (cleavage stage) and Day 5 (blastocyst stage).
Day 5 blastocyst transfer: Add 261 days to the transfer date. This is because the embryo is already 5 days old at transfer, so 266 − 5 = 261 days remain until the estimated due date from fertilisation.
Day 3 cleavage transfer: Add 263 days to the transfer date. The embryo is 3 days old at transfer, so 266 − 3 = 263 days remain.
IVF due dates are generally considered more precise than LMP-based dates, since there is no guesswork about when ovulation or fertilisation occurred. The equivalent LMP date (sometimes called the "paper LMP") is calculated as the transfer date minus 17 days for Day 5, or minus 15 days for Day 3 — this is used by some clinical systems to harmonise IVF records with standard gestational age calculations.
It is worth noting that even with IVF, only about 5% of babies arrive on the exact due date. The useful window is 37–42 weeks, with most healthy IVF pregnancies delivering within this range just like naturally conceived pregnancies.
What Week of Pregnancy Am I In? — Gestational Age Explained
Gestational age is measured in weeks and days from the first day of your last menstrual period (LMP). So "6 weeks pregnant" means 6 weeks have elapsed since your LMP — your actual embryo is only about 4 weeks old at this point, since fertilisation typically occurs around day 14 of the cycle.
To find your current gestational age, simply count the number of days from your LMP to today, then divide by 7. For example, if your LMP was 10 weeks and 3 days ago, you are 10 weeks and 3 days pregnant (written as 10+3 in clinical shorthand).
Our calculator shows your current gestational age automatically if today's date is after your LMP. Key thresholds to know:
- Week 6: Heartbeat typically detectable on transvaginal ultrasound
- Week 12: End of highest miscarriage risk period; nuchal scan window
- Week 20: Anatomy (anomaly) scan; sex often determinable
- Week 24: Viability threshold — survival possible with intensive care
- Week 28: Start of third trimester; glucose tolerance test
- Week 37: Full term — lungs mature, baby ready for delivery
- Week 40: Estimated due date (Naegele's rule)
- Week 42: Post-term — induction usually offered
About This Pregnancy Due Date Calculator
MindSnap's due date calculator uses clinically validated methods to estimate your baby's arrival. For LMP-based calculations it applies Naegele's rule with cycle-length adjustment; for conception dates it uses the standard 266-day embryonic period; and for IVF transfers it uses embryo-age-specific offsets (261 days for Day 5, 263 days for Day 3).
This tool is for informational purposes only. Due dates are estimates — not predictions. Always confirm your due date with a qualified healthcare provider or midwife, who will use ultrasound scanning alongside your menstrual history to establish the most accurate gestational age for your individual pregnancy.
The trimester dates and milestone weeks displayed are based on standard ACOG definitions: Trimester 1 runs from the LMP to the end of week 13 (day 91); Trimester 2 from week 14 to the end of week 26 (day 182); and Trimester 3 from week 27 to delivery. Key scan dates (20 weeks anatomy scan) and viability thresholds (24 weeks) are indicative and vary by country and individual clinical circumstances.