One of the first questions after a positive pregnancy test is also one of the most confusing: "How far along am I?" The answer is a number in weeks, counted from the first day of your last menstrual period — not from the day you conceived. If you want the number right now, use our pregnancy week calculator: enter the first day of your last period and it will tell you your current week, your trimester, and your estimated due date. Keep reading and you will learn exactly how that number is calculated, why the counting starts before conception, how to convert weeks into months, and why an ultrasound sometimes gives a different date.
Pregnancy is measured in gestational age, and gestational age is measured in completed weeks. When someone says they are "14 weeks pregnant," they mean 14 full weeks have passed since the first day of their last menstrual period (LMP). A full-term pregnancy lasts about 40 weeks from that same starting point. That 40-week figure is an average, not a deadline: only a small share of babies arrive on their exact due date, and most arrive within a couple of weeks on either side of it.
Gestational age in weeks = the number of days since the first day of your last period, divided by 7. Estimated due date = first day of your last period + 280 days (about 40 weeks). So if your last period started 98 days ago, you are 98 ÷ 7 = 14 weeks pregnant.
Calculate Your Week NowDoctors date a pregnancy from the first day of your last menstrual period, even though conception happens roughly two weeks later. The reason is practical: the first day of your last period is a date most people can remember or look up, while the exact day of conception is usually impossible to pin down. Ovulation happens around the middle of a typical cycle, sperm can survive in the body for several days, and implantation happens later still. Anchoring the count to the last period gives every pregnancy a consistent, verifiable starting line.
Your result is usually written as weeks plus days — for example, 14 weeks and 3 days, often shown as 14w3d. The "days" part matters more than it looks. At 14w0d you have just completed 14 full weeks; at 14w6d you are one day away from being 15 weeks pregnant. Appointments, screening windows, and trimester boundaries are all scheduled off this weeks-and-days figure, so it is worth recording both numbers rather than rounding to a whole week.
It also helps to know that a due date is an estimate, not an appointment. The medical term is the estimated date of delivery (EDD), and it marks the point at which the pregnancy reaches 40 weeks. Babies born from 37 weeks through 42 weeks are generally considered to have arrived within the normal full-term window. Your care team will track your progress against the EDD while treating it as the center of a range, not a single day you must hit.
You need only two dates: the first day of your last menstrual period and today. Follow these steps:
Say the first day of your last period was March 3, and today is June 9. Count the days: March 3 to March 31 is 28 days, April adds 30 days (58 total), May adds 31 days (89 total), and June 1 through June 9 adds 9 more days (98 total). Divide 98 by 7 and you get exactly 14 — so you are 14 weeks pregnant. Add 280 days to March 3 and your estimated due date is December 8. You can check your own dates the same way with our due date calculator, which runs the same arithmetic for you.
| Your Last Period Started | Days Since Then | How Far Along | Estimated Due Date |
|---|---|---|---|
| March 3 | 98 days | 14 weeks, 0 days | December 8 |
| January 15 | 70 days | 10 weeks, 0 days | October 22 |
| February 10 | 140 days | 20 weeks, 0 days | November 17 |
| April 1 | 196 days | 28 weeks, 0 days | January 6 |
The table above shows the pattern: every 7 days moves you forward exactly one week, and every due date lands 280 days after the last period started. If your own numbers do not land on a round week, the remainder is simply your extra days — 101 days is 14 weeks and 3 days, not "about 14 weeks."
This is the single most confusing part of pregnancy dating, so it is worth spelling out. In a typical 28-day cycle, ovulation happens around day 14, and conception happens around ovulation. But gestational age starts counting on day 1 of the cycle — the first day of your last period, roughly two weeks earlier. So on the day of conception, you are already considered about 2 weeks pregnant, even though the pregnancy itself has just begun.
Nothing is wrong with this picture, and nobody is suggesting you were pregnant before conception. The convention exists because the last period is observable and conception is not. The two-week head start is simply baked into the 40-week figure: about 38 weeks pass from conception to birth, and about 40 weeks pass from the last period to birth. Both describe the same pregnancy.
Where this trips people up is in early pregnancy. A positive test at "4 weeks pregnant" usually means conception happened about 2 weeks ago — right on schedule for a missed period. If you conceived through fertility treatment, your clinic may date the pregnancy from the egg retrieval or transfer date instead, and then convert that to gestational age. Either way, the goal is the same: a consistent count that your care team can use to time screenings and checkups.
The classic method for estimating a due date is Naegele's rule, named after the German obstetrician Franz Naegele. It has a simple form: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. That is the same as adding 280 days, or 40 weeks, to the last period.
Work it through with the earlier example. Last period: March 3. Add one year: March 3 of the following year. Subtract three months: December 3. Add seven days: December 10. That is very close to the exact 280-day count, which gives December 8 — the small difference arises because calendar months vary in length (28 to 31 days), while Naegele's shortcut treats them as uniform. The rule is an excellent estimate, but for the exact calendar date it is best to let a calculator or your clinician do the precise day count. Our due date calculator handles the calendar correctly, including leap years.
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, the rule needs adjusting. A common adjustment is to add the difference between your cycle length and 28 days. If your cycles run 35 days — 7 days longer than average — your due date shifts about 7 days later than the standard Naegele result, because ovulation (and therefore conception) likely happened about a week later than day 14. If your cycles run 21 days, the due date shifts about 7 days earlier. Mention your typical cycle length at your first prenatal visit so your provider can apply this adjustment.
Pregnancy months do not line up neatly with calendar months, because a calendar month is about 4.33 weeks long, not exactly 4. That is why two charts can disagree by a week. The most widely used mapping groups weeks like this:
| Month of Pregnancy | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | Weeks 5–8 | First |
| Month 3 | Weeks 9–13 | First |
| Month 4 | Weeks 14–17 | Second |
| Month 5 | Weeks 18–21 | Second |
| Month 6 | Weeks 22–26 | Second |
| Month 7 | Weeks 27–30 | Third |
| Month 8 | Weeks 31–35 | Third |
| Month 9 | Weeks 36–40 | Third |
So at 20 weeks you are in month 5, and at 32 weeks you are in month 8. If a friend tells you she is "6 months pregnant" at 26 weeks, that matches this chart. Doctors and midwives almost always talk in weeks rather than months precisely because months are fuzzy — weeks give an exact, unambiguous number. When you are filling out forms or booking screenings, always use the week count.
Trimesters divide the 40 weeks into three stages. The most common convention, and the one our pregnancy trimester calculator uses, is: first trimester weeks 1–13, second trimester weeks 14–27, third trimester weeks 28–40. Some sources shift the boundaries by a few days (for example, ending the first trimester at 13 weeks and 6 days), but the practical difference is small, and your provider will tell you which convention they follow.
Each trimester has a different character. The first trimester covers conception, implantation, and the formation of the baby's major organs — which is why fatigue, nausea, and food aversions often peak here. The second trimester is often the most comfortable stretch: energy returns for many people, and the anatomy scan typically happens around 18 to 22 weeks. The third trimester is about growth: the baby gains weight steadily (often around 0.5 lb (about 0.2 kg) per week near the end), and sleep and mobility get harder as the due date approaches.
It is common for an early ultrasound to shift your due date by a few days, and that does not mean anyone made a mistake. Ultrasound dating measures the baby — most accurately the crown-rump length (CRL), the distance from the top of the head to the bottom of the torso — and converts that measurement into a gestational age. The last-period method estimates from your cycle. The two methods answer slightly different questions, so small disagreements are expected.
In the first trimester, ultrasound dating is at its most accurate, typically within about 5 to 7 days. That is why the first-trimester scan is often used to confirm or set the due date. Later scans are less precise for dating, because babies grow at increasingly different rates as pregnancy progresses. A third-trimester scan can estimate the baby's size and weight, but it is a poor tool for resetting the due date.
Clinicians generally follow a rule of thumb: if the ultrasound estimate and the last-period estimate differ by more than a few days in early pregnancy, the ultrasound date usually wins, because it is measuring the baby directly. If they differ by only a day or two, the original date stands. If your due date changes after a scan, ask your provider what the new date is and what it is based on, so your records stay consistent. A moved due date can shift the timing of screenings and tests, so it is worth understanding rather than just noting.
There are a few other reasons dating can shift. If your cycles are irregular, the last-period method is less reliable, because ovulation may not have happened on day 14. If you conceived while coming off hormonal birth control, your last bleed may not have been a true period. And if you are carrying twins or multiples, growth patterns differ, and your provider will monitor dating more closely. In all of these cases, the early ultrasound is the tiebreaker.
Not everyone has a clear last-period date, and that is normal. If your periods are irregular, if you were breastfeeding, if you recently stopped birth control, or if you simply do not track your cycles, your provider will lean on the early ultrasound to date the pregnancy. Bring whatever you do remember — the approximate month, a calendar, or your period-tracking app — because even a rough date helps cross-check the scan.
If your cycles are irregular but you know your typical range, you can still make a rough estimate. Use the midpoint of your usual cycle length to adjust the standard calculation, and treat the result as a window rather than a single date. For example, if your cycles run 32 to 38 days, your ovulation likely happened later than day 14, so your true gestational age may be a few days less than the last-period count suggests. Your provider can refine this at your first visit.
Your gestational age is not just trivia — it drives the schedule of your care. Blood tests and screenings have specific week windows: for example, the nuchal translucency screening typically happens between 11 and 14 weeks, and the anatomy scan between 18 and 22 weeks. Miss a window and the test may be less informative or unavailable. Keeping your week count accurate helps you and your provider book everything at the right time.
The week count also frames what is happening developmentally. Around week 6, the embryo's heart begins beating. By week 12, all major organs have formed. Around week 20, you are halfway, and many people begin to feel movement. Around week 28, the third trimester begins and appointments typically become more frequent. None of these milestones requires you to memorize embryology — but knowing roughly where you are helps the weeks feel less abstract and makes it easier to ask informed questions at each visit.
If you are tracking weight gain alongside your weeks, our pregnancy weight gain chart shows the typical ranges by pre-pregnancy BMI, which can help you see whether your gain is tracking normally for your stage.
Calculating your weeks is straightforward, but some situations need prompt medical attention rather than a calculator. Contact your provider right away if you have heavy vaginal bleeding (soaking a pad in an hour or less), severe abdominal pain, a sudden gush of fluid, a high fever, or if you notice a marked decrease in the baby's movement later in pregnancy. Also call if you are unsure of your dates and have not yet had a dating scan — early dating is most accurate, so an early visit pays off.
Disclaimer: This article is for general information only and is not medical advice. Pregnancy dating, due dates, and screening decisions should be confirmed with your doctor or midwife. If you have concerns about bleeding, pain, or your pregnancy dates, contact your healthcare provider promptly.
Written by the Live Fitness Check editorial team · Reviewed against our calculator methodology · Last updated October 2026
