The 40-week pregnancy calendar is a convention built on a systematic inaccuracy — gestational age is counted from the last menstrual period (LMP), approximately 2 weeks before fertilisation, which means a "40-week pregnancy" is biologically only about 38 weeks of actual fetal development, and ultrasound dating in the first trimester is actually more accurate than LMP dating for most people
The previous articles on this site covered due date calculation methods, why most babies don't arrive on their due date, trimester milestones and screening, twin pregnancy specifics, maternity leave planning, and why the LMP convention makes you seem further along than you are. This article addresses late pregnancy and post-dates decisions — specifically what the research says about induction timing, why due dates have an asymmetric uncertainty range, and the specific populations for whom due date accuracy matters most clinically.
The asymmetric uncertainty of the due date
"Due date" implies precision that doesn't exist. The 40-week due date is actually the median of a gestational length distribution — half of uncomplicated singleton pregnancies deliver before it, half after. But the distribution is not symmetric:
Gestational length distribution for spontaneous labour (uncomplicated singleton):
- 37-38 weeks: approximately 20% of spontaneous deliveries
- 39 weeks: approximately 25%
- 40 weeks: approximately 25%
- 41 weeks: approximately 20%
- 42+ weeks: approximately 10% (with intervention, this is lower)
The range: "full term" is currently defined as 39-40 weeks 6 days; "early term" is 37-38 weeks 6 days; "late term" is 41 weeks 0 days to 41 weeks 6 days; "post-term" is 42+ weeks.
The clinical implication of the asymmetric range: the due date is not a cliff — outcomes for uncomplicated pregnancies at 39-41 weeks are broadly similar. The post-40-week period carries gradually increasing risk, not a sudden increase on day 281.
Ultrasound dating accuracy: why the first trimester is the gold standard
LMP-based dating assumes: ovulation on day 14 of a 28-day cycle, consistent cycle length, accurate LMP recall. All three assumptions are often violated.
Ultrasound dating: measures the fetus directly. First-trimester crown-rump length (CRL) measurement at 7-13 weeks provides dating accuracy of approximately ±5 days (95% confidence interval). This is significantly more accurate than LMP dating for:
- People with irregular cycles
- Those who can't reliably recall their LMP
- Cycles that don't conform to the 28-day/day-14-ovulation model
- Conceptions during postpartum amenorrhoea (before regular periods resumed after a previous pregnancy)
When ultrasound disagrees with LMP by more than 7 days, most clinical guidelines recommend changing the due date to the ultrasound estimate. This is because the first trimester scan is more accurate than LMP calculation for the reasons above.
Second and third trimester ultrasound dating is less accurate — fetal size variation increases as development progresses. A third-trimester ultrasound has dating accuracy of approximately ±3 weeks, making it unreliable for changing a due date established by first-trimester scan.
Naegele's Rule and its modifications
Naegele's Rule (the standard due date formula: add 9 months and 7 days to LMP, or equivalently add 280 days) was formulated by Franz Karl Naegele in 1812 — based on the assumption of a 28-day cycle with day-14 ovulation.
The modification for longer cycles: people with consistently longer cycles (e.g., 35-day cycles with ovulation around day 21) are systematically assigned a due date 7 days earlier than their actual conception date implies. Some obstetricians manually adjust using cycle length: for each day the cycle exceeds 28 days, add one day to the due date.
The IVF exception: for IVF pregnancies, the date of embryo transfer is known precisely. Dating from embryo transfer (adding 38 weeks minus the embryo age at transfer) is more accurate than LMP or ultrasound for IVF. A day-5 blastocyst transfer → add 38 weeks − 5 days = 261 days from transfer.
Post-dates induction: what the evidence says
The major clinical question in late pregnancy is whether to induce labour at 41 weeks or wait for spontaneous labour. The ARRIVE trial (2018) and subsequent research has shifted clinical guidance:
ARRIVE trial finding: elective induction at 39 weeks 0 days (for low-risk, nulliparous patients at term) did not increase caesarean section rates and was associated with a modest reduction in some adverse outcomes compared to expectant management (waiting for spontaneous labour up to 42 weeks).
NICE guidelines (UK, 2021): recommend offering induction between 41 weeks and 42 weeks, with shared decision-making.
The risk gradient after 40 weeks: stillbirth risk increases with gestational age beyond 40 weeks, but from a very low baseline. The absolute risk at 41 weeks is still very low (approximately 1.5-2 per 10,000 per week); the relative increase is approximately 50-100% above the 40-week baseline — which sounds large but remains a small absolute risk.
Due date accuracy for gestational age-dependent decisions
Gestational age accuracy becomes most critical when clinical decisions depend on it:
Preterm delivery decisions: in a threatened preterm delivery at 27 weeks, accurate gestational age determines whether the baby is 27 weeks or 26 weeks — a clinically significant difference for surfactant therapy decisions, steroid administration timing, and intensive care discussions.
Late preterm assessment: babies born at 34-36 weeks ("late preterm") have specific management protocols based on gestational age. A baby believed to be 36+5 who is actually 35+5 (due to inaccurate dating) may be managed differently.
This is why first-trimester dating is prioritised — gestational age established with ±5-day accuracy in the first trimester provides a reliable baseline for all subsequent gestational-age-dependent decisions.
How to use the Due Date Calculator on sadiqbd.com
- LMP-based calculation: the standard Naegele's Rule calculation — most accurate when cycle length is reliably 28 days with ovulation on day 14
- Adjust for cycle length: if your cycle is consistently longer or shorter than 28 days, note that the LMP-based due date may be systematically off — a first-trimester ultrasound provides more accurate dating
- IVF dating: use transfer date as the input if this option is available — the calculator's IVF mode (or manual calculation: transfer date + 261 days for a day-5 embryo) is more accurate than LMP for IVF conceptions
Frequently Asked Questions
If my due date passes with no labour, when should I be concerned? Most obstetric guidelines recommend contacting your midwife or obstetric team if you go 7-10 days past your due date without labour. In many countries (including the UK under NICE guidelines), monitoring (biophysical profiles, CTG, amniotic fluid assessment) begins at 41 weeks, and a conversation about induction is offered between 41 and 42 weeks. Going past your due date is very common — approximately 50% of pregnancies do. What changes is the frequency of monitoring and the recommendation to discuss induction. Going significantly post-dates (42+ weeks) carries gradually increasing risks and most providers would recommend induction before 42 weeks in most clinical scenarios.
Is the Due Date Calculator free? Yes — completely free, no sign-up required.
Try the Due Date Calculator free at sadiqbd.com — calculate your estimated due date from LMP, ultrasound, or IVF transfer date.
