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Pregnancy Due Date Calculator

Estimate when your baby is due using your last period, conception date, or IVF transfer date. Based on Australian clinical guidelines.

Your details

days
21283545

Estimated due date

8 November 2026

Due window: 1 November 2026 – 15 November 2026

Gestational age

10w 0d

Trimester

1st trimester

Days remaining

210

Term category

Preterm
0w13w28w40w
1st trimester2nd trimester3rd trimester

This calculator provides an estimated due date only. It is not a substitute for professional medical advice. Your actual due date may differ based on ultrasound measurements and clinical assessment. Always consult your midwife, GP, or obstetrician. If you have concerns, call Pregnancy, Birth and Baby on 1800 882 436.

What Is a Pregnancy Due Date?

The estimated due date (EDD) is the date your baby is expected to be born, approximately 40 weeks or 280 days from the first day of your last menstrual period.[6] It is also called the estimated date of delivery or estimated date of confinement. It is an estimate, not a precise prediction. Think of it as the centre of a delivery window rather than a fixed appointment.

Only about 4 to 5 percent of babies are born on their exact due date.[5] Around 60 percent are born within one week of their due date, and nearly 90 percent are born within two weeks.[5, 15] Normal pregnancy ranges from 37 to 42 weeks according to the World Health Organisation, giving a five-week window in which a healthy delivery is expected.

How Is a Due Date Calculated?

Naegele's Rule (standard method)

EDD = LMP + 280 days

Developed by German obstetrician Franz Naegele (1778–1851), this is the default clinical calculation used worldwide.[1] It assumes a 28-day cycle with ovulation on day 14. Only about 13 percent of women actually have a textbook 28-day cycle, which is why cycle-length adjustment matters.[1, 5]

Cycle-length adjustment

EDD = LMP + 280 + (cycle length − 28) days

If your cycle is longer or shorter than 28 days, ovulation likely occurs earlier or later. The luteal phase (post-ovulation) is relatively constant at about 14 days, so the variation is in the follicular phase.[1] A 32-day cycle shifts the due date 4 days later; a 25-day cycle shifts it 3 days earlier.

Conception/ovulation date method

EDD = conception date + 266 days

For women who tracked ovulation via basal body temperature, ovulation predictor kits, or fertility monitors. Removes the uncertainty of cycle-length estimation entirely.

IVF transfer method

Day 3: transfer + 263 days  |  Day 5: transfer + 261 days

IVF dates are generally more accurate because fertilisation timing is known precisely. The formulas derive from 266 days from fertilisation minus the embryo age at transfer.[3] RANZCOG advises that IVF-based dates should only be adjusted with extreme caution.

Ultrasound dating

First-trimester crown-rump length (CRL) measurement at 8 to 13 weeks is the gold standard for confirming gestational age, with accuracy of ±5 to 7 days.[2]Accuracy decreases in the second trimester (±10 to 14 days) and third trimester (±21 days). Australian guidelines from RANZCOG and ASUM recommend adjusting the LMP-based due date when first-trimester ultrasound differs by more than 7 days.[2, 3, 4]

Pregnancy Term Categories

In 2013, ACOG and the Society for Maternal-Fetal Medicine redefined “term” pregnancy into four categories because research showed health outcomes differ meaningfully week by week.[8] Babies born at 39 to 40 weeks have the lowest rates of adverse outcomes.[16]

CategoryGestational age
PretermBefore 37+0 weeks
Early term37+0 to 38+6 weeks
Full term39+0 to 40+6 weeks
Late term41+0 to 41+6 weeks
Post-term42+0 weeks and beyond

In Australia, 91 percent of babies are born at term (37 to 41 weeks), with 60 percent born full term (39 to 41 weeks) and 31 percent born early term (37 to 38 weeks). About 8.4 percent are born preterm, before 37 weeks. The median gestational age in Australia has been stable at 39 weeks since 2004.[9]

What Affects How Long Pregnancy Lasts?

A landmark 2013 NIH study by Jukic et al. measured pregnancy length from actual ovulation date rather than LMP. They found the median pregnancy length from ovulation to birth was 268 days (38 weeks, 2 days), with a natural variation of up to 37 days even after excluding preterm births.[7] This is far more variation than most people expect.

  • Maternal age. Older mothers tend to have slightly longer pregnancies.[7]
  • Parity. First-time mothers average about 5 days longer than women who have given birth before. The Mittendorf-Williams study found a median of 288 days for first births versus 283 for subsequent ones.[13]
  • Previous pregnancy length. Women who had longer pregnancies before tend to have longer ones again.[7]
  • Implantation timing. Embryos that took longer to implant also had longer gestations.[7]
  • Smoking. Babies born to mothers who smoked were more likely to be preterm: 14 percent versus 7.8 percent among non-smokers in Australia.[9]

Interestingly, the Jukic study did not find significant associations between pregnancy length and BMI, alcohol consumption, or offspring sex.[7]

Due Dates and Pregnancy in Australia

In 2023, 285,305 babies were born in Australia at a birth rate of 52 per 1,000 women, the lowest since AIHW tracking began in 1998.[10] The median gestational age was 39 weeks. The average maternal age at birth was 31.3 years, up from 28.9 in 1998, with 28 percent of women giving birth aged 35 or older.[10]

Induction of labour trends

The labour induction rate in Australia increased from 25 percent in 2010 to 33 percent in 2023. Spontaneous labour decreased from 56 percent to 40 percent over the same period.[11] Top reasons for induction include diabetes (15%), pre-labour rupture of membranes (11%), and prolonged pregnancy (10%). Only about 13 percent of women in NSW reach 41 completed weeks.[12]

Australian guidelines recommend offering induction between 41 and 42 weeks for low-risk pregnancies.[12] Perinatal mortality approximately doubles between 40 and 42 weeks, though absolute risk remains low. Induction at 41 or more weeks is associated with fewer perinatal deaths without increasing caesarean rates.[12]

Limitations of Due Date Calculations

While due date calculators are a useful starting point, they have important limitations that affect how the result should be interpreted.

  • Precision. Only 4 to 5 percent of babies are born on the exact due date. Natural pregnancy length varies by up to 37 days even when measured from ovulation.[7]
  • Cycle assumptions. Naegele's Rule assumes a 28-day cycle, but cycle lengths normally range from 21 to 35 or more days.[1]
  • Unreliable LMP dates. LMP dating is unreliable when periods are irregular, the woman was recently on hormonal contraception, she does not remember her LMP, or she has conditions like PCOS.[4, 6]
  • Ultrasound margin. Even first-trimester ultrasound has a margin of error of approximately 5 to 7 days. Due dates calculated from LMP have a standard deviation of about 16 days; ultrasound-based dates have a standard deviation of about 14 days.[2]

A better framing: the “due window”

Rather than fixating on a single date, think of your due date as the midpoint of a roughly five-week window (37 to 42 weeks) where healthy birth is normal. Many midwives and researchers advocate using an “estimated due period” of 39 to 41 weeks to reduce unnecessary anxiety.[14]

Pregnancy Timeline by Trimester

1st trimester

Weeks 1–12

Gestational age counted from the first day of LMP. Dating ultrasound recommended at 10 to 13+6 weeks. CRL measurement is most accurate for establishing the due date.[2, 3]

2nd trimester

Weeks 13–27

Anatomy scan typically at around 20 weeks. Ultrasound dating accuracy decreases to ±10 to 14 days. Fetal movements are typically felt from 16 to 22 weeks.

3rd trimester

Weeks 28–40+

Ultrasound dating least accurate (±21 days). Baby is considered early term from 37 weeks, full term from 39 weeks, late term from 41 weeks, and post-term from 42 weeks.[8]

References

  1. 1.Naegele’s Rule and the length of pregnancy — a review. Mongelli M. J Matern Fetal Neonatal Med, 2020. pubmed.ncbi.nlm.nih.gov/33079400/
  2. 2.ASUM/RANZCOG Guidelines for First Trimester Ultrasound. Mizia K et al. Australas J Ultrasound Med, 2018. onlinelibrary.wiley.com/doi/full/10.1002/ajum.12102
  3. 3.Routine Antenatal Assessment. Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG). ranzcog.edu.au/wp-content/uploads/Routine-Antenatal-Assessment.pdf
  4. 4.Estimating Due Date guideline. NSW Health, South Eastern Sydney Local Health District. www.seslhd.health.nsw.gov.au/sites/default/files/documents/Estimating_%20Due_%20Date.pdf
  5. 5.Evidence on Due Dates. Evidence Based Birth. evidencebasedbirth.com/evidence-on-due-dates/
  6. 6.Calculating a Due Date. Johns Hopkins Medicine. www.hopkinsmedicine.org/health/wellness-and-prevention/calculating-a-due-date
  7. 7.Length of human pregnancy and contributors to its natural variation. Jukic AM et al. Hum Reprod, 2013; 28(10): 2848–2855. pmc.ncbi.nlm.nih.gov/articles/PMC3777570/
  8. 8.Definition of Term Pregnancy — Committee Opinion No. 579. American College of Obstetricians and Gynecologists (ACOG), 2013. pubmed.ncbi.nlm.nih.gov/24150030/
  9. 9.Australia’s Mothers and Babies: Gestational Age. Australian Institute of Health and Welfare (AIHW). www.aihw.gov.au/reports/mothers-babies/australias-mothers-babies/contents/baby-outcomes/gestational-age
  10. 10.Australia’s Mothers and Babies: Summary. Australian Institute of Health and Welfare (AIHW). www.aihw.gov.au/reports/mothers-babies/australias-mothers-babies/contents/summary
  11. 11.National Core Maternity Indicators: Induction of Labour. Australian Institute of Health and Welfare (AIHW). www.aihw.gov.au/reports/mothers-babies/national-core-maternity-indicators/contents/labour-and-birth-indicators/induction-of-labour
  12. 12.Management of Pregnancy Beyond 41 Weeks. NSW Health. www1.health.nsw.gov.au/pds/ActivePDSDocuments/GL2014_015.pdf
  13. 13.Better way to predict pregnancy term. Mittendorf R et al. University of Chicago. chronicle.uchicago.edu/961107/pregnancy.shtml
  14. 14.Constructing the uncertainty of due dates. Res Sociol Health Care, 2015. pmc.ncbi.nlm.nih.gov/articles/PMC4390179/
  15. 15.What Week You’re Most Likely to Give Birth. ParentData by Emily Oster. parentdata.org/what-week-youre-most-likely-to-give-birth/
  16. 16.What Is Full-Term?. March of Dimes. www.marchofdimes.org/find-support/topics/pregnancy/what-full-term

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Frequently Asked Questions

Common questions about pregnancy due dates, how they are calculated, and what to expect in Australia.

How accurate is a due date calculator?

Due date calculators based on your last menstrual period are accurate to within about two weeks for most pregnancies. A first-trimester ultrasound narrows this to approximately five to seven days. Only 4 to 5 percent of babies arrive on the exact calculated date, but about 60 percent arrive within one week and roughly 90 percent within two weeks. IVF-based calculations are generally the most accurate because the fertilisation date is known precisely.

Can my due date change during pregnancy?

Yes. If a first-trimester ultrasound (8 to 13 weeks) differs from the LMP-based estimated due date by more than seven days, Australian guidelines from RANZCOG recommend using the ultrasound date instead. In the second trimester, the due date may be adjusted if the discrepancy exceeds 10 to 14 days. For IVF pregnancies, dates should only be adjusted with extreme caution. Your care provider will review dating at your first ultrasound appointment.

What if I have irregular periods?

Standard LMP-based dating is less reliable when cycles are irregular. The best option is an early dating ultrasound, ideally between 8 and 13 weeks, which can estimate gestational age regardless of cycle regularity. Women recently off hormonal contraception may also have unreliable LMP dates. If you have PCOS or other conditions that affect ovulation, ultrasound dating is the recommended approach.

Is the due date different for first pregnancies?

Evidence suggests first-time mothers tend to carry slightly longer. The Mittendorf-Williams study found first-time mothers averaged 288 days from their LMP compared to 283 days for subsequent pregnancies, a difference of about five days. This is because the follicular phase can be slightly longer in first pregnancies. Some adjusted formulas account for this, though most standard calculators do not.

How is the due date calculated for IVF pregnancies?

IVF due dates are more precise because the fertilisation timing is known. For a Day 3 embryo transfer, the estimated due date is the transfer date plus 263 days. For a Day 5 blastocyst transfer, it is the transfer date plus 261 days. These formulas are based on 266 days from fertilisation, minus the embryo age at the time of transfer. RANZCOG advises that IVF-based dates should only be adjusted with extreme caution even if an ultrasound measurement differs slightly.

What happens if I go past my due date?

Going past 40 weeks is common. About one in three pregnancies in Australia involve induction of labour. Late term begins at 41 weeks and zero days, and post-term begins at 42 weeks. Australian guidelines recommend offering induction between 41 and 42 weeks for low-risk pregnancies. From 41 weeks, your care provider will typically offer increased monitoring including CTG and ultrasound. In New South Wales, only about 13 percent of women reach 41 completed weeks of pregnancy.

What is the most common week to give birth?

Week 39 is the most common week for delivery overall. Approximately 57 to 60 percent of babies are born between 39 and 40 weeks. First-time mothers are most likely to deliver in weeks 39 to 40. About 70 percent of babies are born before their due date at 40 weeks and zero days. Around 8.4 percent of babies in Australia are born preterm, before 37 weeks.

Does maternal age affect the due date?

Older mothers tend to have slightly longer pregnancies. The Jukic et al. study found a statistically significant association between maternal age and pregnancy length. The average maternal age in Australia is now 31.3 years, up from 28.9 in 1998, and 28 percent of Australian mothers are aged 35 or older. Advanced maternal age is also associated with higher rates of gestational diabetes, pre-eclampsia, and induction of labour.

Should I think of my due date as a single day or a range?

Evidence strongly supports thinking of it as a range or due window rather than a single day. Normal full-term birth spans 37 to 42 weeks, a five-week window. The Jukic et al. study found natural variation of up to 37 days even in uncomplicated pregnancies measured from ovulation. Many midwives and researchers advocate using an estimated due period, for example 39 to 41 weeks, rather than fixating on a single date. The single due date can cause unnecessary anxiety when it passes.

How do twins or multiples affect the due date?

Twin pregnancies are typically shorter, with average delivery around 36 to 37 weeks. The due date is calculated the same way initially, but clinical management differs. For dating twins, RANZCOG and ASUM recommend using the crown-rump length of the larger twin. Most twin pregnancies are delivered by 37 to 38 weeks depending on chorionicity. Triplets and higher-order multiples deliver even earlier on average.