How Is a Due Date Calculated?
Your estimated due date (EDD) is one of the first things your doctor or midwife will establish. Most people assume it's calculated from conception — but that's not quite right. The standard method, known as Naegele's Rule, calculates from the first day of your last menstrual period (LMP):
Due Date = First Day of LMP + 280 days (40 weeks)
The 40 weeks begins from LMP rather than conception because ovulation typically occurs around day 14 of a 28-day cycle — meaning conception is approximately two weeks after the LMP start date. You can work out your EDD instantly using our Pregnancy Due Date Calculator.
How Accurate Is Your Due Date?
Here's the honest answer: your due date is an estimate. Research shows that only about 4–5% of babies are born on their exact due date. Most arrive within a two-week window on either side.
A normal, full-term pregnancy is considered 37–42 weeks. Approximately 70% of pregnancies deliver between 39 and 41 weeks. Factors affecting the accuracy of your EDD include:
- Cycle length: Naegele's Rule assumes a 28-day cycle. If your cycles are longer or shorter, conception likely occurred at a different point
- Uncertain LMP date: Many people don't recall their LMP precisely
- Dating ultrasound: An ultrasound performed at 8–14 weeks (the dating scan) is more accurate than LMP calculation and will typically update your EDD if there's a discrepancy of more than 5–7 days
Your care provider may revise your due date after the first trimester dating scan. This is completely normal and doesn't indicate a problem.
Pregnancy Week by Week: Key Milestones
Use our Pregnancy Weeks Calculator to see exactly where you are in your pregnancy. Here's a summary of key milestones by trimester:
First Trimester (Weeks 1–12)
- Weeks 1–2: Technically part of pregnancy by convention, but conception hasn't occurred yet
- Week 4: Missed period; home pregnancy test typically positive
- Week 6–8: First heartbeat detectable by transvaginal ultrasound
- Week 8–10: All major organs and body structures are forming — the embryo becomes a fetus
- Week 10–12: Nuchal translucency (NT) scan and blood screening for chromosomal conditions
- Week 12: Risk of miscarriage drops significantly; most couples announce pregnancy
Second Trimester (Weeks 13–26)
- Week 16–20: Anatomy (morphology) ultrasound scans baby's organs, spine, and limbs in detail
- Week 18–22: Most mothers feel baby's first movements (quickening)
- Week 20: Halfway point; baby is roughly 25cm head-to-rump
- Week 24: Threshold of viability — babies born after this point have a meaningful chance of survival with intensive neonatal care
Third Trimester (Weeks 27–40+)
- Week 28: Third-trimester check-ins begin; gestational diabetes screening (if not done at 24–28 weeks)
- Week 32–34: Baby is generally head-down by this stage
- Week 36: Weekly midwife or obstetrician appointments typically begin
- Week 37: Baby is considered early term; lungs are mature
- Week 40: Estimated due date
- Week 41–42: Post-term monitoring begins; induction may be offered
What to Do After Getting Your Due Date
Once you have an EDD, a few practical steps make the months ahead much smoother:
- Register with a care provider early: Obstetric and midwifery services fill quickly — especially popular public hospital programs. Many recommend registering by 8–10 weeks
- Book your dating scan: Aim for 11–13 weeks for the most useful first trimester information
- Start prenatal vitamins if you haven't: Folate and iodine are recommended from preconception; continue throughout pregnancy
- Understand your Australian healthcare options: Public hospital (Medicare-covered), shared care with your GP, or private obstetric care each have different booking timelines and costs
As you prepare, you'll also want to track your baby's development after birth. Our Baby Age Calculator makes it easy to see your child's age in weeks and months — useful for developmental milestone tracking.
A beautiful way to document the journey is a dedicated pregnancy journal. Browse pregnancy journals on Amazon AU — week-by-week formats are particularly lovely for capturing memories.
Twins and Multiple Pregnancies
Due date calculation is the same for multiple pregnancies, but your EDD is more likely to be revised after ultrasound. Twin pregnancies are typically delivered earlier — often at 36–38 weeks. Your care provider will discuss the specific timing considerations for your situation.