Pregnancy 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.