Finding out you're pregnant is an exciting time, and one of the first questions is "when is my baby due?" If you know your conception date, you can calculate your due date more accurately than using the standard last menstrual period (LMP) method. This guide explains exactly how to calculate your due date and what to expect throughout your pregnancy.
Calculating Due Date from Conception
If you know when you conceived, the calculation is straightforward:
Due Date = Conception Date + 266 days (38 weeks)
Example Calculation
Conception date: 15th March 2025
Add 266 days: 15th March + 266 days
Estimated due date: 6th February 2026
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How Conception Date Relates to LMP
The NHS and most healthcare providers date pregnancy from the first day of your last menstrual period (LMP), not conception. This is because:
- Most women know their LMP but not their exact conception date
- Ovulation typically occurs around day 14 of a 28-day cycle
- This means conception usually happens about 2 weeks after LMP
Due Date = LMP + 280 days (40 weeks)
Conversion:
Conception Date ≈ LMP + 14 days
Why Conception Date Matters
Knowing your conception date can give a more accurate due date if:
- You have irregular periods
- You conceived through IVF (exact date known)
- You were tracking ovulation
- You only had intercourse on specific dates
Pregnancy Timeline by Weeks
Dating Scan Accuracy
The NHS dating scan (at 8-14 weeks) is the most accurate way to determine your due date:
| Method | Accuracy | Notes |
|---|---|---|
| Early scan (8-14 weeks) | ± 3-5 days | Most accurate method |
| LMP calculation | ± 2 weeks | Assumes 28-day cycle |
| Conception date | ± 3-5 days | If date is certain |
| Later scans (20+ weeks) | ± 1-2 weeks | Less accurate due to size variation |
If your dating scan shows a different due date (more than 5-7 days from LMP calculation), the NHS will use the scan date for all future appointments and calculations.
When Do Most Babies Arrive?
Understanding when babies actually arrive helps set realistic expectations:
| Timing | Percentage |
|---|---|
| On the exact due date | 4-5% |
| Within 1 week of due date | ~50% |
| Within 2 weeks of due date | ~80% |
| Before 37 weeks (preterm) | ~8% |
| After 42 weeks (post-term) | ~5% |
Factors Affecting Due Date
Several factors can influence when you actually deliver:
- First vs subsequent pregnancy: First babies often arrive later
- Multiple pregnancy: Twins typically arrive 36-37 weeks
- Maternal age: May influence timing
- Previous birth history: Pattern may repeat
- Medical conditions: May require earlier delivery
Important Pregnancy Milestones
| Week | Milestone |
|---|---|
| Week 8-14 | Dating scan, nuchal translucency screening |
| Week 12 | End of highest miscarriage risk period |
| Week 18-21 | Anomaly (20-week) scan |
| Week 24 | Viability milestone |
| Week 28 | Third trimester, anti-D if Rh negative |
| Week 32 | Appointments become more frequent |
| Week 36 | Baby likely in head-down position |
| Week 37 | Considered "early term" |
| Week 39-40 | "Full term" |
| Week 41 | "Late term" - induction discussed |
| Week 42 | "Post term" - induction usually offered |
IVF Due Date Calculation
For IVF pregnancies, the calculation is precise because exact dates are known:
Due Date = Transfer Date + 263 days
From 5-Day Blastocyst Transfer:
Due Date = Transfer Date + 261 days
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1-12 | All major organs form, highest risk period |
| Second | 13-27 | Rapid growth, movements felt, anatomy scan |
| Third | 28-40 | Weight gain, preparation for birth |
What If Your Cycle Isn't 28 Days?
Standard calculations assume a 28-day cycle with ovulation on day 14. If your cycle is different:
- Shorter cycles (e.g., 25 days): You may have ovulated earlier, so baby may arrive sooner than LMP suggests
- Longer cycles (e.g., 35 days): You may have ovulated later, so baby may arrive later than LMP suggests
- Irregular cycles: Dating scan will be most reliable
NHS Care Timeline
Based on your due date, you'll receive scheduled care:
- 8-12 weeks: Booking appointment with midwife
- 8-14 weeks: Dating scan and screening tests offered
- 16 weeks: Routine appointment
- 18-21 weeks: Anomaly scan
- 25 weeks (first pregnancy): Routine appointment
- 28 weeks: Routine + blood tests
- 31 weeks (first pregnancy): Routine appointment
- 34 weeks: Routine appointment
- 36 weeks: Routine + baby position check
- 38 weeks: Routine appointment
- 40 weeks: Routine + discussion if overdue
- 41 weeks: Membrane sweep offered, induction discussed
Understanding Pregnancy Dating and Due Date Calculations
Calculating a due date from conception is one of the most searched pregnancy-related topics, yet it is widely misunderstood. The standard method used by the NHS and healthcare providers worldwide is Naegele's rule, which calculates the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). However, if you know your conception date, the calculation is different: add 266 days (38 weeks) from conception, since fertilisation typically occurs about 14 days after the start of the last period.
It is important to understand that a "due date" is really an estimated date. Only about 4-5% of babies are born on their exact due date. The normal range for a full-term pregnancy is 37 to 42 weeks, meaning your baby could arrive up to three weeks before or two weeks after the calculated date and still be considered within normal range. First-time mothers tend to go slightly past their due date on average, while subsequent pregnancies may be slightly shorter.
The NHS divides pregnancy into three trimesters, each with distinct developmental milestones. The first trimester (weeks 1-12) covers the period of greatest risk for miscarriage and is when all major organs begin forming. The second trimester (weeks 13-27) is often considered the most comfortable period, when morning sickness typically subsides and the baby's movements become noticeable. The third trimester (weeks 28-40) sees rapid growth, with the baby gaining about 230 grams per week in the final months.
NHS Antenatal Care Timeline
Knowing your due date helps you plan your antenatal appointments with the NHS. You should book your first midwife appointment (booking appointment) as early as possible, ideally before 10 weeks. The dating scan at 8-14 weeks provides the most accurate due date estimate using ultrasound measurements of the baby's crown-rump length. This scan date may adjust your due date by up to two weeks compared to LMP-based calculations. The anomaly scan at 18-21 weeks checks the baby's physical development in detail.
Additional appointments are scheduled at regular intervals throughout pregnancy, with increased frequency in the final weeks. First-time mothers typically have around 10 appointments, while those who have given birth before may have around 7. These appointments monitor blood pressure, urine, baby's growth, and position, helping to identify any complications early.
Planning Around Your Due Date
Once you have an estimated due date, use it as a guide for practical preparations rather than a firm deadline. Aim to have your hospital bag packed by 36 weeks, complete any nursery preparations by 34-36 weeks, and notify your employer about maternity leave plans well in advance. UK employment law requires you to notify your employer by the 15th week before the expected week of childbirth (around 25 weeks pregnant), though many women choose to inform their employer earlier.
Statutory Maternity Pay (SMP) in the UK provides 90% of your average weekly earnings for the first 6 weeks, followed by the lower of £184.03 per week or 90% of your average weekly earnings for the remaining 33 weeks. You are entitled to take up to 52 weeks of maternity leave in total. Plan your finances around these dates, as the transition from full pay to SMP represents a significant change in household income for many families.
How accurate is a due date calculated from conception?
If you know the exact date of conception, adding 266 days gives a reasonable estimate, but individual variation means this is still approximate. The dating ultrasound scan at 8-14 weeks is considered the most accurate method, typically accurate to within plus or minus 5 days. After 14 weeks, ultrasound dating becomes progressively less accurate. Factors like cycle length, time of ovulation, and individual growth patterns all introduce variability.
Can my due date change during pregnancy?
Yes, your due date may be adjusted after your dating scan if the ultrasound measurements differ significantly from the LMP-based calculation (typically by more than 5-7 days). After this early scan, the due date is generally not changed again, even if later scans suggest the baby is measuring larger or smaller than expected, as growth variation in later pregnancy does not reliably indicate a different due date.
What happens if I go past my due date?
Going past your due date is common, especially for first pregnancies. The NHS will offer a membrane sweep at 40 weeks (first pregnancy) or 41 weeks to try to stimulate natural labour. If labour has not started by 41-42 weeks, induction will typically be recommended, as the risk of complications increases after 42 weeks. You will be offered increased monitoring with cardiotocography (CTG) and ultrasound to check the baby's wellbeing while you wait.
How does the NHS calculate a due date differently from conception-based calculators?
What prenatal care does the NHS provide after my due date is confirmed?
Can I choose my place of birth on the NHS and how does my due date affect planning?
Dr. Sarah Chen, PhD
Research Scientist, Public Health
Sarah holds a PhD in Public Health from the University of Edinburgh and has published research on UK health metrics and obesity trends. She translates complex medical data into practical, accessible guidance for everyday readers.