Your due date isn't calculated from the month you conceived — it's calculated from the first day of your last menstrual period (LMP), or confirmed by an early ultrasound. That's why a scan can say you're "4 weeks pregnant" within days of a recent marriage or a single instance of intercourse, and why the month you conceived only gives you an approximate due date, not an exact one.
Why your scan date can feel confusing at first
Pregnancy is medically dated worldwide from the first day of your LMP, not from the date of marriage, intercourse, or conception. In a typical 28-day cycle, ovulation happens around day 14 and fertilisation follows shortly after — so by the time of a scan roughly two weeks later, the pregnancy is already dated as 4 weeks, because those two weeks before ovulation are counted in. A 4-week pregnancy does not mean conception happened 4 weeks ago. It reflects the standard method of calculating pregnancy age, not how recently you conceived. Understanding this one point clears up a lot of unnecessary anxiety, confusion — and sometimes unfair judgment of couples who conceived quickly after marriage.
A month-by-month approximate due date guide
If you know roughly when you conceived, here's an approximate guide to your due date — a full-term pregnancy lasts about 40 weeks from your LMP, which works out to roughly 9 months and a week from the actual month of conception:
| Month you conceived | Approximate due month |
|---|---|
| January | Around October |
| February | Around November |
| March | Around December |
| April | Around January |
| May | Around February |
| June | Around March |
| July | Around April |
| August | Around May |
| September | Around June |
| October | Around July |
| November | Around August |
| December | Around September |
This is only an approximate estimate. Your actual due date is calculated from your last menstrual period and/or confirmed on an early ultrasound — not from the conception month alone.
How pregnancy weeks map to the nine months
Pregnancy is medically tracked in weeks because it gives a more precise picture of your baby's growth and milestones than months do — but since most people think in months day-to-day, here's roughly how the two line up across a 40-week pregnancy:
| Month | Approximate weeks |
|---|---|
| Month 1 | Weeks 1–4 |
| Month 2 | Weeks 5–8 |
| Month 3 | Weeks 9–13 |
| Month 4 | Weeks 14–17 |
| Month 5 | Weeks 18–22 |
| Month 6 | Weeks 23–27 |
| Month 7 | Weeks 28–31 |
| Month 8 | Weeks 32–35 |
| Month 9 | Weeks 36–40 |
This is why your doctor may say you're "28 weeks" when you'd have counted yourself as "7 months" — both describe the same point in pregnancy, just in different units, and the week-based figure is simply the more precise of the two.
Why the ultrasound is the more reliable reference point
However you counted the months, an early ultrasound remains the most reliable way to confirm how far along you are and firm up your due date — especially if your cycles are irregular or you're unsure of your LMP. For the full schedule of scans across pregnancy and what each one confirms, see how many ultrasound scans you need in pregnancy.
Frequently asked
Is my due date calculated from when I conceived?
No. It's calculated from the first day of your last menstrual period, or confirmed by an early ultrasound — not from your conception date, marriage date, or a single instance of intercourse.
Why did my scan say I was already "4 weeks pregnant" so early?
Because pregnancy dating counts from your LMP, which includes roughly two weeks before ovulation and fertilisation even happen. It's a dating convention, not a sign that conception happened earlier than you thought.
How accurate is a due date estimated from the conception month alone?
It's a rough guide only, generally accurate to within a few weeks. Your LMP and an early ultrasound give a far more precise estimate.
Why does my doctor talk in weeks instead of months?
Weeks give a more precise picture of your baby's stage of development and the timing of key scans and milestones. Months are a convenient way to talk about it day-to-day, but weeks are the medical standard.