How to Calculate Your Due Date and Gestational Age
Six methods Canadian obstetricians and midwives use to date your pregnancy — from the 200-year-old Naegele's Rule to a first-trimester dating ultrasound. Plus when you can find out your baby's sex.
Try Our Calculator Skip to the MethodsGestational age vs. fetal age — why the numbers don't match
The first thing that confuses almost every newly-pregnant person is the math: pregnancy is described in weeks, but the count starts two weeks before conception. That's because clinicians measure from a date you remember (your last period) rather than a date you can't pinpoint (conception). The result is two different "ages" that describe the same pregnancy.
Gestational age
Measured from the first day of your last menstrual period (LMP). This is what every prenatal appointment, ultrasound report, and pregnancy app uses. A pregnancy is "term" at 40 weeks gestational age.
LMP + 280 days
Fetal age (conceptional age)
Measured from the date of conception — usually about two weeks after the LMP, around ovulation. Used in IVF and embryology research. A 10-week gestational pregnancy is roughly 8-week fetal age.
Conception + 266 days
The six methods used to date a pregnancy
Canadian clinicians don't pick one method — they use a hierarchy. A first-trimester ultrasound trumps LMP if the two disagree by more than five days. If you had IVF, the transfer date trumps everything. Here's how each method works.
Naegele's Rule (LMP method)
The original due-date formula, developed in 1830 by German obstetrician Franz Naegele. Take the first day of your last menstrual period, subtract 3 months, add 7 days and 1 year. Assumes a regular 28-day cycle with ovulation on day 14.
First-trimester ultrasound (CRL)
Measures the crown-rump length (CRL) of the embryo, typically at 8–12 weeks. Per SOGC guidelines, a CRL measurement between 7 and 84 mm dates a pregnancy within ±5 days. If this disagrees with LMP by more than the threshold, the ultrasound date becomes the official EDD.
Known conception date
If you tracked ovulation with LH strips, basal body temperature, or a cycle app and know the date of conception, add 266 days. Equivalent to LMP + 280 days only if your cycle is exactly 28 days; for longer or shorter cycles, conception-date dating is more accurate.
IVF transfer date
The most precise method available. Embryo age at transfer (typically Day 3 or Day 5) is known to the hour. Your fertility clinic will calculate an EDD that supersedes both LMP and dating ultrasound, with sub-day accuracy.
Fundal height measurement
Your healthcare provider measures the distance in centimetres from your pubic bone to the top of your uterus (the fundus). Between weeks 20 and 36, this number roughly equals your gestational age in weeks. Used as a check on growth, not as a primary dating method.
Second-trimester ultrasound
Measures biparietal diameter (BPD), head circumference, abdominal circumference, and femur length. Accuracy widens from ±5 days in early first trimester to ±10–14 days by the second trimester. Used when first-trimester dating wasn't performed.
Not sure how far along you are?
Use our free eligibility calculator to see whether you're at 7 weeks yet — the earliest you can find out your baby's sex with an EarlyReveal at-home DNA test.
Pregnancy by trimester — what's happening when
Pregnancy is divided into three trimesters that don't quite match calendar months. Here's the 40-week journey at a glance.
The 40-week pregnancy timeline
Key dating milestones during pregnancy
These are the moments when your due date may be confirmed, refined, or — occasionally — revised.
Method comparison — which is most accurate?
If two methods disagree, here's the order of priority Canadian clinicians follow (per SOGC guideline):
| Method | When to use | Accuracy | Priority |
|---|---|---|---|
| IVF transfer date | Assisted reproduction only | ±0–1 day | Highest |
| First-trimester ultrasound (CRL) | Week 7–13 | ±5 days | Override LMP |
| Known conception date | Tracked ovulation | ±3–5 days | Override LMP |
| Naegele's Rule (LMP) | Regular 28-day cycle | ±7–10 days | Default starting point |
| Second-trimester ultrasound | Week 14–21 | ±7–10 days | When 1st-tri not done |
| Fundal height | Week 20–36 | ±2–3 weeks | Growth check, not dating |
When can I find out my baby's sex?
The most common question that follows "how far along am I?" is "when can I find out if it's a boy or girl?" In Canada, you have three options:
- At 7+ weeks — at-home DNA gender test (EarlyReveal). Detects the Y chromosome from a few drops of your blood. 99.9% accuracy.
- At 10+ weeks — NIPT (non-invasive prenatal testing). Covered by provincial health plans only for high-risk pregnancies; otherwise $500–$900 private.
- At 18–22 weeks — anatomy ultrasound. Free under public health coverage; 95–98% accurate.
The Canadian context — what to expect from your clinician
In Canada, the Society of Obstetricians and Gynaecologists of Canada (SOGC) recommends every pregnant person be offered a dating ultrasound between 8 and 12 weeks. This is covered by OHIP (Ontario), RAMQ (Québec), MSP (BC), AHCIP (Alberta), and other provincial plans. Your family doctor or midwife will refer you. If you live in Québec, your pregnancy will be tracked through the Programme québécois de dépistage prénatal, which offers a first-trimester combined screening between weeks 11 and 13+6 days for those who choose it.
Frequently asked questions
How accurate is my pregnancy app's due date?
Apps like Flo, Ovia, and What to Expect use Naegele's Rule (LMP + 280 days). They're accurate for people with regular 28-day cycles but can be off by 1–2 weeks for longer or irregular cycles. A first-trimester ultrasound is more reliable.
What if my periods are irregular?
If your cycles are longer than 32 days, shorter than 24 days, or unpredictable, LMP-based dating will likely be inaccurate. Your doctor will rely on a first-trimester dating ultrasound to set your EDD.
Why does my ultrasound date differ from my LMP date?
Ovulation timing varies cycle to cycle. If you ovulated earlier or later than day 14, your "real" gestational age won't match the LMP calculation. SOGC says: if the difference is more than 5 days in the first trimester, use the ultrasound date.
Can my due date change?
Yes — usually once, after the first-trimester dating ultrasound. After that, the EDD is fixed even if later ultrasounds suggest a different size. Re-dating in late pregnancy is rare and only happens in specific clinical situations.
How many weeks pregnant am I if I conceived three weeks ago?
About 5 weeks gestational, 3 weeks fetal. Gestational age = conception age + 2 weeks. The clock starts ticking 2 weeks before you were biologically pregnant.
Can I take an EarlyReveal test before week 7?
No. Before week 7, there isn't enough fetal DNA circulating in your bloodstream for reliable Y-chromosome detection. Taking the test earlier risks a false negative.
Is 40 weeks really how long pregnancy lasts?
It's the average. Only about 4% of babies are born exactly on their due date. A pregnancy is "term" from 37 weeks onward, and "post-term" past 42 weeks. Most spontaneous deliveries fall between 39 and 41 weeks.
Does EarlyReveal tell me how far along I am?
No — EarlyReveal determines fetal sex, not gestational age. For accurate dating, see your physician or midwife for a first-trimester ultrasound. Our eligibility calculator estimates whether you've reached the 7-week threshold for testing.
Ready to find out — boy or girl?
Once you've hit 7 weeks gestational age, you can find out your baby's sex from the comfort of home — with the only Health Canada MDEL-licensed at-home gender test in Canada, processed at our Laval lab.
Order Your TestThis article is for informational purposes and does not replace personalized medical advice. For accurate dating of your pregnancy, consult your physician, midwife, or provincial prenatal screening program. EarlyReveal is not a diagnostic test; it determines fetal sex only. Sources: Society of Obstetricians and Gynaecologists of Canada (SOGC) clinical practice guidelines; Public Health Agency of Canada; Programme québécois de dépistage prénatal (MSSS Québec).
