Pregnancy Due Date: How It's Calculated and What to Expect Week by Week

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.

Common Mistakes When Calculating Your Due Date

Even with clear guidance, a few common errors trip up expectant parents when working out their EDD. Understanding these pitfalls helps you communicate more accurately with your care provider and manage your own expectations.

Counting From the Wrong Date

The most frequent mistake is counting from the day you think conception occurred, or the day you had intercourse. While this seems logical, pregnancy dating doesn't work this way in medical practice. The 40-week calculation always starts from your LMP, not from ovulation or conception day.

If you have a 28-day cycle and conceived around day 14, you're already considered "two weeks pregnant" at conception. This can feel confusing, but it's the universal standard used across all Australian maternity services.

Forgetting to Account for Irregular Cycles

Naegele's Rule assumes ovulation at day 14 of a 28-day cycle. If your cycles are regularly 35 days, you likely ovulated around day 21 — a full week later. This means your actual due date could be a week later than a basic LMP calculation suggests.

Similarly, if you have polycystic ovary syndrome (PCOS) or irregular periods, LMP dating becomes much less reliable. In these cases, your dating ultrasound at 8–13 weeks becomes the gold standard. Don't be surprised if your doctor revises your EDD significantly after this scan.

Misunderstanding What "40 Weeks" Means

When people hear "40 weeks pregnant," many assume this means 10 months. It's actually closer to nine calendar months, because pregnancy weeks are measured as full seven-day periods. A month averages 4.3 weeks, not exactly four.

This confusion leads some parents to panic when they're "41 weeks" thinking they're nearly 10.5 months pregnant. You're not — you're about 9.4 calendar months, which is within the normal full-term range.

Relying Only on Apps Without Professional Confirmation

Pregnancy apps and online calculators (including ours) are helpful tools, but they can't replace your dating ultrasound. Many parents become attached to the date an app gives them, then feel anxious when their 12-week scan moves it by five or six days.

Trust the ultrasound measurement. Early pregnancy scans measuring crown-rump length are accurate to within 3–5 days and are more reliable than LMP calculations, especially before 14 weeks.

When Your Due Date Might Be Adjusted: Real Scenarios

Your EDD isn't set in stone. Several situations trigger a revision, and it's helpful to understand when and why this happens.

IVF and Fertility Treatment Pregnancies

If you conceived through IVF or other assisted reproductive technology, your due date is calculated differently and with greater precision. For a fresh embryo transfer, the due date is calculated from the egg retrieval date plus 266 days (38 weeks), because the exact fertilisation date is known.

For a frozen embryo transfer, the calculation accounts for the embryo's age at transfer. A day-5 blastocyst transfer, for example, uses the transfer date plus 261 days. Your fertility clinic will provide your obstetrician with the exact EDD, which typically won't be revised unless there's a significant growth discrepancy on ultrasound.

Dating Scan Shows Major Discrepancy

If your 12-week ultrasound shows your baby measuring 13 weeks and 4 days based on crown-rump length, your doctor will likely update your due date to match. The general rule: if the difference is more than 5–7 days from your LMP-based date, the scan measurement takes priority.

This doesn't mean something is wrong. It usually just means your ovulation was earlier or later than assumed, or your LMP date wasn't quite right. Once adjusted after the dating scan, your EDD rarely changes again unless there's a significant growth concern later.

Uncertain or Unknown LMP

Some women don't track their periods closely, were on hormonal contraception recently, or conceived while breastfeeding with irregular cycles. In these cases, there's no reliable LMP to use as a baseline.

Your doctor will rely entirely on ultrasound dating. The earlier the scan, the better — an ultrasound at 8–9 weeks is remarkably accurate. If you don't have an LMP date, make sure your care provider knows early so they can arrange appropriate dating scans.

Managing Expectations: Overdue Babies and Induction

Most Australian maternity services become more watchful once you pass your due date. Understanding the timeline and options helps you make informed decisions if you go past 40 weeks.

What Happens at 40 Weeks

Your due date arrives and... often nothing happens. This is completely normal. Remember, only 4–5% of babies arrive on their exact EDD. Your midwife or obstetrician will typically schedule an appointment within a few days to check your wellbeing and the baby's position and heartbeat.

Most Australian hospitals don't intervene at 40 weeks unless there's a medical indication. You'll be offered monitoring, not immediate induction.

Post-Dates Monitoring: 41 Weeks Onwards

Between 40 and 41 weeks, monitoring increases. You'll likely be offered:

• Non-stress test (CTG): Monitors baby's heartbeat and movement patterns for 20–40 minutes
• Amniotic fluid assessment: Ultrasound checks there's adequate fluid around the baby
• Discussion about membrane sweep: A vaginal examination where your midwife or doctor gently separates the amniotic sac from the cervix, which can trigger natural labour

Most public hospitals in NSW, Victoria, and Queensland follow similar protocols: offer a membrane sweep at 40–41 weeks, then discuss induction around 41 weeks if labour hasn't started.

Induction Timing and Your Choices

Current Australian guidelines, based on evidence from studies including the ARRIVE trial, typically recommend offering induction between 41 and 42 weeks for low-risk pregnancies. Many hospitals specifically recommend induction at 41 weeks plus a few days (around 41+3 to 41+5).

This timing balances two risks: inducing too early (when the cervix may not be favourable and induction is more likely to fail) versus waiting too long (when placental function can decline and stillbirth risk, while still very low, begins to increase slightly).

You have the right to decline induction and continue monitoring. If you choose expectant management past 41 weeks, you'll typically be offered monitoring every 2–3 days. Most hospitals strongly encourage delivery by 42 weeks due to increased risks beyond this point.

Regional Variations in Post-Dates Care

While national guidelines are broadly similar, individual hospitals have their own protocols. For example:

• Some Melbourne public hospitals routinely offer induction at 41+0 weeks
• Rural and regional hospitals in Queensland and Western Australia may discuss induction earlier (around 40+5 to 41+0) due to distance from tertiary facilities
• Private obstetricians often have different thresholds — some routinely induce at 40 weeks, particularly for women over 35 or those with previous pregnancy complications

Always ask your specific care provider what their approach is. There's no single "right" answer — it depends on your individual circumstances, preferences, and risk factors.