The moment you confirm you are pregnant, the doctor often says "you are 6 weeks along" — yet it feels like you just conceived. Where does pregnancy week counting actually start? How is the due date calculated? Many parents-to-be stare at "8 weeks + 3 days" on their chart in confusion. Today we explain the two core algorithms clearly, so you can understand every number on your checkup sheet and estimate when your baby will arrive.
How Gestational Age Is Counted: LMP Is the Standard
In medicine, gestational age is counted from the first day of the last menstrual period (LMP), not from the day of conception. Why? Because ovulation and conception are hard to pinpoint, while the first day of your last period is the easiest date to remember. A full pregnancy is counted as 40 weeks (280 days): the first two weeks occur before conception actually happens, which typically takes place around week 3.
Example: if your LMP was March 1 and today is May 1, you are around 8 weeks and 5 days along (61 days from March 1 to May 1; 61 ÷ 7 = 8 weeks plus 5 days). The "weeks + days" notation on your chart simply means the completed weeks since LMP plus the remaining days. When the ultrasound gives a more precise read, the doctor uses that corrected figure.
If your cycle is regular (28 days), the LMP method is reliable. If your cycle is irregular or you cannot recall your LMP, use the ovulation method — ovulation typically occurs about 14 days before the next period, and conception happens on ovulation day or within 1-2 days. Counting back two weeks from ovulation gives your gestational starting point, with an error of roughly 3-5 days. Use our ovulation calculator to work out your fertile window and cross-check both methods.
| Method | Best For | Starting Point | Error Range |
|---|---|---|---|
| LMP method | Regular cycle (28±7 days) | First day of last period | ±1-2 weeks |
| Ovulation method | Irregular cycle / known ovulation | Ovulation day − 14 days | ±3-5 days |
| Ultrasound correction | Everyone (most accurate early) | Back-calculated from crown-rump length | ±3-7 days |
Of the three, the first-trimester ultrasound (8-13 weeks) is the most authoritative for dating. The doctor adjusts your due date based on it, and all subsequent checkup dates follow this corrected gestational age.
How the Due Date Is Calculated: Naegele's Rule and Ultrasound
The most common formula for estimating a due date is Naegele's rule: take the LMP, subtract 3 months (or add 9), then add 7 days. Example: LMP March 1 → month 3 − 3 = 0, so add 9 → December; day 1 + 7 = 8; due date is December 8 of the same year. Another example: LMP November 20 → month 11 + 9 = 20 → August of the following year; day 20 + 7 = 27; due date is August 27 the next year.
This rule assumes a 28-day cycle with ovulation on day 14. For women with longer or shorter cycles, the estimate drifts — with a 35-day cycle, ovulation is about a week later and the due date should shift later by about a week as well. The more accurate approach combines ultrasound: an early scan measuring the crown-rump length (CRL) gives a precise gestational age, and the doctor corrects the due date accordingly. If your period is regular and your LMP is certain, the two methods usually agree within a few days; the ultrasound-adjusted date is the one to follow.
Many parents also want to know which trimester they are in: the first trimester ends at 13 weeks + 6 days, the second runs from 14 to 27 weeks + 6 days, and the third starts at 28 weeks. Each stage has completely different checkup items — the schedule below maps them for you.
| Weeks | Trimester | Key Checkup | Notes |
|---|---|---|---|
| 6-8 weeks | First | First ultrasound, confirm heartbeat | Confirm intrauterine pregnancy |
| 11-13 weeks | First | Nuchal translucency (NT) scan | Do not miss the window |
| 15-20 weeks | Second | Down syndrome screening / NIPT | Choose per doctor advice |
| 20-24 weeks | Second | Anatomy (anomaly) scan | Systematic structural screening |
| 24-28 weeks | Second | Glucose tolerance test | Screens gestational diabetes |
| 28-32 weeks | Third | Follow-up scan + fetal position | Monitor position and movement |
| 36+ weeks | Third | Weekly checkups | Fetal monitoring + delivery planning |
If you are still planning and want to lock in your fertile window, start with the ovulation calculator to find your ovulation day, then count back to estimate gestational age. To pin down exact calendar dates for each checkup milestone, the age calculator helps you convert week spans into actual calendar days so you never miss an appointment.
Delivering Around the Due Date: 40 Weeks Is Not a Deadline
Many parents assume the baby must arrive exactly on the due date. In fact, only about 5% of babies are born on their due date. A full-term pregnancy is defined as 37 to 42 weeks, and giving birth within two weeks on either side of the due date is completely normal. Before 37 weeks is preterm; after 42 weeks is post-term — both trigger extra monitoring or intervention.
On a 280-day (40-week) schedule, the natural window for labor to begin is roughly weeks 38-42. If your due date passes with no sign of labor, the doctor typically arranges fetal monitoring and amniotic fluid checks; after 41 weeks, induction may be recommended. Rather than worrying about "going past the due date," focus on regular checkups and tracking fetal movement — if movement is regular and your checkup results are normal, most babies arrive naturally between weeks 38 and 41.
FAQ
Q1: Is gestational age counted from the day of conception?
No. Gestational age is counted from the first day of the last menstrual period; conception actually happens around week 3. If your LMP was March 1 and conception occurred March 14, that date is already "about 1 week and 6 days" in gestational terms, so there is no need to track the conception date — just remember your LMP.
Q2: What if I cannot recall my last period?
Use ultrasound dating. A scan between 6 and 13 weeks measuring the embryo or crown-rump length estimates gestational age within 3-7 days — the most reliable substitute when LMP is unknown. The ovulation method also works: count back 14 days from ovulation as your starting point.
Q3: Will my due date be off if my cycle is irregular?
Yes. Naegele's rule assumes a 28-day cycle; with a 35-day cycle, ovulation is later and the formula estimates an earlier due date by about a week. In this case the ultrasound-corrected date is authoritative, and the doctor reschedules your checkups around it.
Q4: I am past my due date. Should I go to the hospital immediately?
No need to panic. Weeks 40-41 are still normal range; what matters is regular checkups and fetal movement counting. After 41 weeks the doctor evaluates whether to induce, and beyond 42 weeks (post-term) hospital monitoring is required. Go in any time you experience labor signs: regular contractions, bloody show, or water breaking.
Q5: Is the due date calculated the same way for twins?
The starting point is the same, but twins usually deliver early — on average around 36-37 weeks. Your doctor will follow twin-pregnancy management standards and schedule earlier monitoring and a delivery plan, rather than waiting until 40 weeks.