Pregnancy Calculator
Result
Weeks pregnant today
- Days since the last full week
- 5 days
- Total days of pregnancy
- 453 days
- Estimated conception date
- July 21, 2025
- Estimated due date
- April 13, 2026
A pregnancy calculator counts how far along you are: it turns the first day of your last period into a gestational age in weeks and days, a conception date, and an estimated due date. Gestational age is measured from that first day rather than from fertilization, which is why the count begins about two weeks before conception and why weeks 1 and 2 of pregnancy are weeks in which you are not yet pregnant. Enter your cycle length too, and the conception date and due date move while the gestational age stays where it is.
The three trimesters
| Stage | Weeks of pregnancy | What it covers |
|---|---|---|
| First trimester | 0 weeks 0 days to 13 weeks 6 days | The stage in which the major systems and structures form. ACOG takes the second trimester as beginning at 14 weeks 0 days, which is where this boundary sits. |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days | The middle of the three. The first movements a mother can feel, called quickening, usually fall in this stage, and an ultrasound for dating is normally done before it ends. |
| Third trimester | 28 weeks 0 days onwards | The last stage, running to birth. It opens at 28 weeks 0 days because that is where ACOG puts the boundary, and it has no closing week of its own. |
The weeks run from the first day of your last period, matching the count the calculator reports. This is the same frame the result above uses to label the stage.
What develops week by week
| Weeks of pregnancy | What develops |
|---|---|
| Week 1 to 2 | The first week starts with the first day of your period, and you are not yet pregnant. An egg is released from an ovary at the end of the second week, which is when conception is most likely. |
| Week 3 | Sperm travel through the cervix into the fallopian tube, where one meets the egg. That moment is conception, and the combined cell is a zygote. It divides on the way down the tube into a ball of cells called a blastocyst. |
| Week 4 | The blastocyst reaches the uterus and buries itself in the uterine wall, where the lining is thick with blood and ready to support it. It sticks tightly and takes nourishment from the mother's blood. |
| Week 5 | The start of the embryonic period, when all the major systems and structures develop. Cells take on specific functions, blood, kidney and nerve cells appear, and the brain, spinal cord and heart begin to form. |
| Weeks 6 to 7 | Arm and leg buds grow, the brain forms into five areas, and eyes and ears begin to form. The heart beats at a regular rhythm, which a vaginal ultrasound can show, and blood pumps through the main vessels. |
| Week 8 | Arms and legs have grown longer, and hands and feet begin to form and look like little paddles. The brain continues to grow and the lungs start to form. |
| Week 9 | Nipples and hair follicles form, elbows develop as the arms grow, and the toes can be seen. All of the essential organs have begun to grow. |
| Week 10 | Eyelids develop and begin to close, the outer ears take shape, facial features become more distinct, and the intestines rotate. At the end of this week the embryo becomes a fetus, the stage that lasts until birth. |
| Weeks 11 to 14 | The eyelids close and will not reopen until about the 28th week, the face is well formed, and nails appear on fingers and toes. Genitals appear, the liver makes red blood cells, and tooth buds for the baby teeth appear. |
| Weeks 15 to 18 | The skin is almost transparent and fine hair called lanugo develops on the head. Muscle tissue and bones keep developing and the bones harden, the baby begins to move and stretch, and the liver and pancreas produce secretions. |
| Weeks 19 to 21 | The baby can hear, is more active, and floats around. The mother may feel a fluttering in the lower abdomen, called quickening, which is the first time movements are felt from outside. By the end of this stage the baby can swallow. |
| Week 22 | Lanugo covers the whole body, eyebrows and lashes appear, and meconium, the first bowel movement, is made in the intestinal tract. Muscle development increases, and the heartbeat can be heard with a stethoscope. |
| Weeks 23 to 25 | Bone marrow begins to make blood cells, the lower airways of the lungs develop, and the baby starts to store fat. |
| Week 26 | Eyebrows and eyelashes are well formed and all parts of the eyes are developed. Footprints and fingerprints are forming, air sacs form in the lungs, and the baby may startle at loud noises. |
| Weeks 27 to 30 | The brain grows rapidly and the nervous system develops enough to control some body functions. The eyelids can open and close, and the lungs produce surfactant, which helps the air sacs fill with air after birth. |
| Weeks 31 to 34 | The baby gains weight quickly and stores iron, calcium and phosphorus. Rhythmic breathing occurs although the lungs are not fully mature, and the bones are fully developed but still soft. |
| Weeks 35 to 37 | The baby weighs about 5 1/2 pounds (2.5 kilograms) and keeps gaining weight without getting much longer. Fat forms under the skin, sleeping patterns become definite, and the heart, blood vessels, muscles and bones are complete. |
| Week 38 to 40 | Lanugo is gone except on the upper arms and shoulders, fingernails may extend past the fingertips, and head hair becomes coarse. In the 40th week it has been 38 weeks since conception, and the baby could be born any day. |
These stages are counted in gestational weeks, that is from the first day of your last period, so they sit two weeks ahead of the age counted from conception. They describe what is forming, not how large it is; for growth measurements, ask your clinician.
Formula
Gestational age = today − first day of your last period, in days; weeks = days ÷ 7 with the remainder left as days
- LMP
- The first day of your last menstrual period. This is the zero point of gestational age, and it is why the count is about two weeks ahead of the age counted from conception.
- Today
- The date the page is showing. Because the count runs up to today, the number of weeks changes once a week and the number of days changes every day.
- Total days
- The whole number of days between the two, reported on its own so you can check the weeks and days against it. (days)
- Weeks
- Total days divided by seven, rounded down. This is the number most people mean by how far along they are. (weeks)
- Remaining days
- What is left over after the weeks are taken out, so 192 days reads as 27 weeks 3 days rather than as 28 weeks. (days)
- Conception date
- The estimated day of ovulation, placed 14 days before the next period is due. It is not affected by the count up to today.
- Due date
- The first day of your last period plus 280 days, shifted by any difference between your cycle length and 28 days.
Use it when you want the count rather than a calendar of dates. The due date calculator takes the same two inputs and reports the due date and the four stages around it; this page reports where you are now, and it needs today's date to do it. If you already know your due date and want to work backwards, use the conception calculator.
Worked examples
Last period on 7 July 2025, 28-day cycle
- Count the days from 2025-07-07 to 2026-01-15: 192 days.
- 192 ÷ 7 = 27 weeks with 3 days left over, so the gestational age is 27 weeks 3 days.
- 192 days falls in the second trimester, which runs from 14 weeks 0 days to 27 weeks 6 days.
- Conception is placed 14 days after the last period: 2025-07-21, and the due date is 280 days after the last period: 2026-04-13.
The weeks and the days are two views of one number. Add the weeks multiplied by seven to the days and you get the total, which is the figure the trimester is judged on.
Last period on 18 December 2025, 35-day cycle
- 2025-12-18 to 2026-01-15 is 28 days, which is exactly 4 weeks 0 days.
- The cycle is 7 days longer than the assumed 28, so conception moves 7 days later: 2025-12-18 plus 21 days is 2026-01-08.
- The due date moves by the same 7 days: 280 + 7 = 287 days after the last period, which is 2026-10-01.
- The gestational age does not move with them, because it is counted from the last period either way.
This is the case where the two frames pull apart: the same last period gives a gestational age of 4 weeks and a conception date three weeks ago, because ovulation had not happened at the end of the last period.
Last period on 15 January 2026, 28-day cycle
- The last period began today, so the count is 0 days and the gestational age is 0 weeks 0 days.
- Conception is expected 14 days after the last period, which is 2026-01-29 and still in the future.
- The due date is 280 days after the last period: 2026-10-22.
A conception date in the future is the correct answer here, not an error. Weeks 1 and 2 of pregnancy are counted before ovulation, which is the point of the first row of the table below.
Limitations
The count assumes a regular cycle and an accurate memory of when your last period began. Gestational age is a convention rather than a measurement, so an ultrasound before 14 weeks can move your due date and with it the weeks shown here. This page also does not tell you how big your baby is. Size estimates are published as ranges that widen as pregnancy goes on, and the source used for the table below prints only one weight figure in the whole page, so the table describes what develops rather than how large it is. Ask your clinician for growth measurements.
Frequently asked questions
- Why does the count start before I was pregnant?
- Because gestational age is measured from the first day of your last period, not from the day of fertilization. Nobody knows the exact hour conception happens, while the first day of a period is something you can point at. The result is that the first two weeks of pregnancy fall before ovulation, and the count runs about two weeks ahead of the age counted from conception.
- How do I convert weeks pregnant into weeks since conception?
- Subtract two. In your 40th week of pregnancy it has been 38 weeks since conception. The gap is the fortnight between the first day of your last period and ovulation, and it grows if your cycle is longer than 28 days, which is why this page asks for your cycle length.
- What are the three trimesters?
- They divide pregnancy into three parts. The first trimester runs up to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days onwards. This page counts the boundaries from the first day of your last period, which is the same frame as the weeks and days it reports above them.
- Does my cycle length change how many weeks pregnant I am?
- No, and that is deliberate. The number of weeks is the distance from your last period to today, and your cycle length does not change either of those dates. What the cycle length does change is the estimated conception date and the due date, because a longer cycle means ovulation comes later. Use the due date calculator if the date is what you came for.
- My conception date is in the future.
- That happens when the last period you entered is recent. If it began today, the count is 0 days, and ovulation is not expected for another fortnight. The date is not a claim that conception has happened; it is where the line would put it. Weeks 1 and 2 of pregnancy are counted before ovulation, so a due date can exist before a pregnancy does.
- The page says 28 weeks but I feel further along.
- Symptoms and size vary widely, and neither is a dating method. The number here is arithmetic from two dates. If your dates are uncertain, or if your cycles were irregular before you conceived, an ultrasound is the way to settle it: ACOG treats a scan before 14 weeks as accurate enough to revise the estimated due date. Bring the question to your clinician rather than adjusting the dates yourself.
References
- Fetal development (Medical Encyclopedia) — MedlinePlus, United States National Library of Medicine. Source of the 18 development stages in the table below, and of the statement that gestational age is measured from the first day of the last menstrual cycle, so that weeks 1 and 2 are counted before pregnancy begins.
- Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, United States, with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine (May 2017). Source of the 280-day convention, the 28-day cycle it assumes, and the trimester boundaries used to label the result.
- Having a Baby (patient FAQ) — American College of Obstetricians and Gynecologists, United States. The plain-language account of how pregnancy is dated in weeks and why the count starts at the last period rather than at conception.