Free pregnancy calculator
Find out how many weeks pregnant you are in seconds. Enter the first day of your last period and this pregnancy calculator returns your current gestational age in weeks and days, your trimester, and your estimated due date, updated live, as you type.
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Naegele's rule (LMP + 280 days). Due dates are estimates — only ~5% of births occur on the exact date. Always confirm with your healthcare provider.
Results are estimates. Consult a professional.
How pregnancy is dated: gestational age from your last period
When a clinician says you are "14 weeks pregnant," they are quoting your gestational age — and, perhaps surprisingly, they are counting from the first day of your last menstrual period (LMP), not from the day you conceived. This is the universal convention used by doctors, midwives, and every pregnancy app, and it is exactly how this pregnancy calculator works: enter the first day of your last period and it returns how many weeks and days pregnant you are today, which trimester you are in, and your estimated due date.
Dating from the LMP has a quirk worth understanding up front. Because conception happens around two weeks after your period starts, you are counted as roughly "2 weeks pregnant" on the day you actually conceive. A full-term pregnancy is therefore 40 weeks of gestational age — about 38 weeks of actual fetal development plus the two-week head start built into the LMP method.
How many weeks pregnant am I? How the weeks are calculated
"How many weeks pregnant am I?" is the question this calculator exists to answer. The math is simple calendar arithmetic: count the number of days from the first day of your last period to today, then divide by seven. The whole number is your week; the remainder is the extra days, which is why pregnancy is reported as, for example, "14 weeks and 4 days" (often written 14+4).
Because the count runs from your LMP, you start at week 0, not week 1 — the first seven days are "0 weeks and 1–6 days." The calculator updates the moment you enter your date, so you never have to count weeks on a calendar by hand. Enter your LMP once and it shows your current gestational age, your trimester, and your due date together.
The three trimesters of pregnancy
Pregnancy is divided into three trimesters, each lasting roughly three months. The boundaries are defined by gestational week, so once the calculator knows how many weeks pregnant you are, it also knows your trimester. The table below shows the standard week ranges used by ACOG and the NHS.
| Trimester | Gestational weeks | Roughly |
|---|---|---|
| First trimester | Weeks 0–13 | Conception through about month 3 |
| Second trimester | Weeks 14–27 | About months 4 to 6 |
| Third trimester | Weeks 28–40+ | About month 7 to birth |
Standard trimester boundaries by gestational age (ACOG; NHS). Full term is 39–40 weeks; 37–42 weeks is the normal delivery range.
A worked example using the pregnancy calculator
Aisha's last period began on March 1, 2025. As of June 11, 2025 she wants to know how many weeks pregnant she is, which trimester she is in, and her due date. Here is the arithmetic the calculator runs.
- Count the days since the LMP. From March 1 to June 11 is 102 days. That is the raw length of the pregnancy so far in gestational terms.
- Divide by seven to get weeks and days. 102 ÷ 7 = 14 with a remainder of 4, so Aisha is 14 weeks and 4 days pregnant (written 14+4).
- Read off the trimester. At 14 weeks she has just crossed from the first into the second trimester (weeks 14–27).
- Add 280 days for the due date. March 1 plus 280 days lands on December 6, 2025 — her estimated due date.
If Aisha instead wanted only the delivery date, the dedicated due-date calculator isolates that single output; this pregnancy calculator is built around the broader "how far along am I" question.
How the due date is calculated (Naegele's rule)
The estimated due date comes from Naegele's rule, the 200-year-old method still used worldwide: take the first day of the last period and add 280 days, which is 40 weeks. An equivalent shortcut is to add one year, subtract three months, and add seven days to the LMP. Both give the same date the calculator shows.
Conception age vs. gestational age
The single most common source of confusion in pregnancy dating is the two-week gap between gestational age and conception (fetal) age. Gestational age counts from your last period; conception age counts from the day the egg was fertilized — about two weeks later. Whenever you read "weeks pregnant" from a clinician or an app, it is gestational age.
Why count from the LMP at all, when conception is what actually starts the pregnancy? Because almost no one knows the exact day they conceived, whereas most people can name the first day of their last period. The LMP is the one reliable date available, so medicine standardized on it. If you do know your conception or ovulation date, the conception calculator and ovulation calculator work that angle, and the implantation calculator estimates when the pregnancy physically began.
What happens each trimester: a brief week-by-week overview
Knowing your week tells you roughly what is happening with both the pregnancy and your body. This is a high-level map, not medical advice — every pregnancy runs its own course.
First trimester (weeks 0–13)
The major organs and structures form during these weeks — the foundation period. Many people feel the strongest early symptoms now: nausea ("morning sickness"), fatigue, tender breasts, and a missed period that often prompts the first test. The first prenatal appointment and an early dating ultrasound usually fall in this window.
Second trimester (weeks 14–27)
Often called the easier stretch: nausea typically fades, energy returns, and a visible bump appears. The anatomy ultrasound around weeks 18–22 checks development and can reveal the baby's sex. Most people first feel movement ("quickening") somewhere between 16 and 25 weeks.
Third trimester (weeks 28–40+)
The baby gains weight and matures the lungs and brain in preparation for birth. Prenatal visits become more frequent, and symptoms like back pain, swelling, and Braxton Hicks contractions are common. Term begins at 37 weeks, with most births between 39 and 41 weeks.
How accurate is a pregnancy calculator?
A pregnancy calculator gives a well-grounded estimate, not a measurement. The arithmetic itself is exact — counting days from your LMP and adding 280 is the same method your clinician uses. What introduces uncertainty is the assumption underneath the LMP method: that your cycle is a regular 28 days and that you ovulated on day 14. Real cycles vary, so two people with the same LMP but different cycle lengths are not actually the same number of weeks along.
- Cycle length matters. A longer cycle means later ovulation, which makes you slightly less far along than the LMP suggests; a shorter cycle, slightly more. The standard formula can't see this.
- LMP recall is imperfect. Research finds only about half of people accurately remember their last period's first day — and a wrong start date shifts every output.
- Ultrasound usually wins. A first-trimester scan that measures the baby (crown–rump length, accurate to within ±5–7 days) is the most reliable dating method, and clinicians will re-date the pregnancy from it if it disagrees with the LMP by more than about a week.
- It is not a diagnosis. The calculator tells you how far along you likely are — it cannot confirm a pregnancy is healthy or progressing normally. Only prenatal care can do that.
When to see a doctor in pregnancy
A calculator tells you how far along you are; it cannot watch over the pregnancy. Book your first prenatal (antenatal) appointment as soon as you have a positive test — ideally before 10 weeks — so dating can be confirmed by ultrasound and early screening can be arranged.
Between visits, contact your midwife, doctor, or maternity unit promptly if you notice any of the following — these warrant urgent assessment rather than waiting:
- Vaginal bleeding or fluid leaking from the vagina.
- Severe or persistent abdominal pain, or regular tightening before 37 weeks.
- A bad headache that won't go away, vision changes, or sudden swelling of the face, hands, or feet — possible signs of pre-eclampsia.
- Reduced or absent baby movements in the third trimester, once movements are established.
- A high temperature, persistent vomiting, or burning when you pass urine.
Data sources and methodology
Gestational age is counted from the first day of the last menstrual period and the estimated due date is the LMP plus 280 days (Naegele's rule) — the standard clinical convention described by the American College of Obstetricians and Gynecologists (ACOG) and the NHS. Trimester boundaries (weeks 0–13, 14–27, and 28–40+) follow the same sources. Where a cycle is not a regular 28 days, or an early ultrasound is available, dating may differ by a few days. All figures here are population-average estimates computed with plain calendar arithmetic from the date you enter.
ACOG — Methods for Estimating the Due Date (Committee Opinion).Frequently asked questions about the free pregnancy calculator
About this pregnancy calculator
This pregnancy calculator runs entirely in your browser. The date you enter never leaves your device — nothing is sent to a server, logged, or shared. It dates the pregnancy from your last menstrual period (gestational age), shows your current weeks-and-days and trimester, applies Naegele's rule (LMP + 280 days) for the due date, and updates instantly on every change.
Calculators Cloud offers 400+ free tools with no sign-up. The whole Health calculators shelf includes due date, ovulation, conception, and implantation tools alongside this one. Or browse the full calculator directory.