Pregnancy Calculator: How It Works
A pregnancy calculator turns one date into a full timeline: how many weeks along you are, which trimester, when key checks usually fall, and the estimated due date. This page explains the counting convention that confuses almost everyone, and what the milestones actually are.
Why week one comes before conception
Gestational age is counted from the first day of your last menstrual period, not from conception. Conception occurs around week two of that count, and a positive test usually appears around week four. So a pregnancy described as 40 weeks involves roughly 38 weeks of actual development.
The convention exists because the LMP is a date most people can identify, while ovulation usually is not. Every clinical milestone, scan window and screening cut-off uses this count, so it is worth getting used to.
The trimesters
| Trimester | Weeks | Broadly |
|---|---|---|
| First | 0 – 13 | Organ formation; nausea and fatigue common |
| Second | 14 – 27 | Symptoms often ease; movement felt; anatomy scan |
| Third | 28 – 40+ | Rapid growth; more frequent monitoring |
Typical milestones
| Week | Commonly happens |
|---|---|
| 4 – 5 | Missed period; positive home test |
| 6 – 8 | First appointment; heartbeat often visible on scan |
| 11 – 14 | Dating scan and first-trimester screening |
| 18 – 22 | Anatomy scan; sex often identifiable |
| 16 – 25 | First movements felt — later in a first pregnancy |
| 24 – 28 | Glucose screening |
| 28 | Appointments typically become more frequent |
| 36 | Position assessed; birth planning discussions |
| 37 – 42 | Term; birth expected |
Local guidelines vary considerably. Your own schedule comes from your midwife or doctor, not from a general timeline.
The due date is a midpoint
Around 4% of babies are born on the estimated date. Roughly half arrive within a week either side, and about 90% within two weeks. Treating the date as a deadline creates unnecessary anxiety in the final fortnight; treating it as the centre of a two-week window is closer to reality.
How dating gets refined
An LMP-based estimate assumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, the date shifts accordingly — add the difference between your cycle length and 28 days. A first-trimester ultrasound is more accurate still, within about five to seven days, because early embryonic growth is highly consistent. Clinicians usually revise the date when a scan differs from the LMP estimate by more than about a week.
Multiples and IVF
Twin and higher-order pregnancies are monitored more closely and typically deliver earlier — often around 36 to 37 weeks for twins — though the due date is still calculated the same way. IVF pregnancies are dated from the transfer, which gives the most precise estimate available: 263 days from a day-3 transfer, 261 from a day-5 blastocyst.
This page is general information, not medical advice. Every pregnancy is different, and your midwife or doctor's assessment takes precedence over any calculator. Contact your maternity care provider promptly about bleeding, severe pain, reduced fetal movement, or any concern about your pregnancy.