Pregnancy Calculator

How the pregnancy calculator counts gestational age from your last period

Gestational age is not counted from the day of conception. Instead, it is counted from the first day of your last menstrual period (LMP), because that date is usually known far more precisely than the day conception actually happened. In a typical cycle, ovulation and conception occur roughly two weeks after the last period starts, which means gestational age is consistently about two weeks ahead of the embryo’s or fetus’s actual developmental age. This is a commonly misunderstood point: a pregnancy described as “6 weeks pregnant” is closer to 4 weeks since conception, not 6.

This calculator takes the LMP date you enter and counts the exact number of whole days between that date and today (or any other reference date you choose), then breaks that day count into whole weeks and a remainder of days, the same “N weeks and D days” format used on a real prenatal chart. The estimated due date is calculated with Naegele’s rule, a widely used method that simply adds 280 days, or 40 weeks, to the first day of the last period. That 280-day figure is itself an average built on a standard 28-day cycle, so an unusually long or short cycle shifts the real due date earlier or later than this estimate.

To calculate gestational age by hand, follow the same three steps the calculator runs.

Two other figures the calculator shows follow from the same day count. Weeks remaining is simply 40 minus the current gestational age in whole weeks, floored at zero so a pregnancy that has already passed the 40-week mark never shows a negative number of weeks left. Percent complete measures progress toward that same 280-day full term as a percentage, capped at 100 percent for the same reason: once the due date has passed, there is no meaningful sense in which a pregnancy is more than fully complete by this particular yardstick, even though it is, in reality, still ongoing until birth. Both numbers are there as a quick, visual sense of where a pregnancy sits along the standard timeline, not as a prediction of exactly when labor will begin.

The three trimesters, and where the boundaries fall

Pregnancy is commonly split into three trimesters, each covering a distinct stretch of development. This calculator uses the boundaries most widely cited by health sources: the first trimester runs from the start of pregnancy through the end of week 13, the second trimester covers week 14 through the end of week 27, and the third trimester runs from week 28 onward, through birth. A pregnancy sitting right at 14 weeks and 0 days has just crossed into the second trimester; one at 13 weeks and 6 days is still, just barely, in the first.

These week ranges are approximate rather than a precise biological cutoff, and different clinics or textbooks occasionally draw the line a day or two differently. What stays consistent is the underlying idea: each trimester groups together a period of pregnancy with broadly similar development milestones and, often, broadly similar symptoms, which is why the trimester framework is used so widely even though its exact boundaries are a convention rather than a hard rule.

The first trimester is generally when early pregnancy symptoms such as nausea and fatigue are most noticeable, and it is also when the risk of early pregnancy loss is highest, which is part of why many people wait until the trimester is nearly over before sharing the news widely. The second trimester is often described as the most comfortable stretch for many people, as early symptoms typically ease and noticeable movement usually begins. The third trimester brings the most rapid weight gain for the baby and is when most of the practical preparation for birth, and most of the closer medical monitoring, tends to happen. None of this is a substitute for what a specific pregnancy actually feels like, since the range of what is normal, at every stage, is wide.

Pregnancy weeks to months: converting the week count

Pregnancy is tracked in weeks, but most people also want the answer in months. The two do not line up neatly: a pregnancy week is exactly 7 days, while an average calendar month is 30.4 days, so a month holds about 4.35 weeks, not 4. Converting is one division, and the table below runs it for every week from 4 to 42.

Read it by finding your week in the first column. At 8 weeks pregnant you have completed 1.8 months and are in the second month. Converting 28 weeks to months gives 6.4, the point where the third trimester begins. Charts that count four-week months give slightly different answers, so a month figure from another source can differ from this one by a few days.

To go the other way, the second table lists where each of the first nine months begins and ends.

Pregnancy term and due date: what “full term” actually means

“Due date” suggests a single day, but the American College of Obstetricians and Gynecologists (ACOG), together with the Society for Maternal-Fetal Medicine (SMFM), actually defines a full six-week window around it, split into four labeled ranges: early term runs from 37 weeks 0 days through 38 weeks 6 days, full term runs from 39 weeks 0 days through 40 weeks 6 days, late term runs from 41 weeks 0 days through 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond. The distinction exists because, on average, outcomes measured across large populations are best within the full-term window specifically, not simply “anytime from 37 weeks on,” which is part of why a doctor may frame 39 weeks, not 37, as the real target for a scheduled delivery that has no medical reason to happen earlier.

The calculated due date itself only marks the center of a normal range, not a deadline. A pregnancy that goes into labor anywhere from late term into the very start of postterm is still common and is not automatically treated as overdue in a concerning sense, though a doctor will usually monitor a pregnancy more closely once it moves past the due date, and will typically discuss induction if a pregnancy continues well into the postterm range.

Detecting a pregnancy: hCG and testing

Both home pregnancy tests and clinical blood tests work by detecting human chorionic gonadotropin (hCG), a hormone produced by the tissue that becomes the placenta almost immediately after implantation. Levels roughly double every two to three days in a typical early pregnancy, which is why testing a few days earlier than expected can produce a faint or false negative even in a real pregnancy: there simply may not be enough hCG built up yet for the test to detect.

Home urine tests are qualitative, meaning they report a plain positive or negative rather than a specific number, and most brands claim reliable detection around the time of a missed period, with some sensitive versions claiming a few days earlier. A clinical blood test ordered by a doctor can be qualitative (positive/negative) or quantitative, reporting an actual hCG concentration, and can generally detect a pregnancy slightly earlier than a home urine test because blood tests are more sensitive to low hormone concentrations. A single low or borderline reading is often followed up with a second test 48 hours later specifically to check whether the level is roughly doubling as expected, which is more informative than one reading on its own. Testing too early, using diluted urine, or misreading a faint line are the most common sources of a confusing result; a doctor can confirm any result that seems inconsistent with how a cycle has been tracking.

Pregnancy management: medication, weight gain, exercise and nutrition

Medication safety. For decades the U.S. Food and Drug Administration (FDA) labeled prescription drug pregnancy risk with a single letter, A, B, C, D or X, from lowest to highest documented concern. Category A meant controlled human studies found no demonstrated risk; Category B meant animal studies found no risk but adequate human studies were lacking, or animal studies found a risk that human data did not confirm; Category C meant animal studies showed a risk, human data was lacking, and the drug might still be used if the potential benefit justified the potential risk; Category D meant human risk evidence existed, but the drug might still be justified in a serious condition; and Category X meant risk clearly outweighed any possible benefit, so the drug was contraindicated in pregnancy. It is worth being current about this system: the FDA formally replaced it for newly approved drug labels in June 2015 with the Pregnancy and Lactation Labeling Rule (PLLR), which requires a narrative risk summary instead of a single letter, because a single letter grade was widely seen as oversimplifying genuinely different kinds of evidence. The old letters still appear constantly in older references, older drug labels, and everyday clinical conversation, which is why they are worth understanding, but the accurate, current source for any specific medication is a doctor or pharmacist checking that drug directly, not a letter grade alone.

Weight gain. Recommended pregnancy weight gain is not one number for everyone; published Institute of Medicine (IOM) guidelines set a different total gain range depending on pre-pregnancy BMI category, from roughly 12.5 to 18 kg for an underweight starting BMI down to roughly 5 to 9 kg for an obese starting BMI, for a single, non-twin pregnancy. A doctor or midwife can apply these published ranges to your own pre-pregnancy height and weight and track your progress against them week by week.

Exercise. For an uncomplicated pregnancy, ACOG guidance generally supports at least 150 minutes of moderate-intensity activity per week, spread across most days, continuing many of the same activities done before pregnancy at a comfortable intensity. Walking, swimming, stationary cycling and prenatal yoga are commonly cited as generally well-tolerated options, while contact sports, activities with a high fall risk, and exercise flat on the back after the first trimester are generally discouraged. Any pre-existing condition, pregnancy complication, or new symptom during exercise is a reason to get individual guidance from a doctor rather than defaulting to general guidance.

Nutrition. Widely cited general guidance during pregnancy includes adequate folic acid (particularly important before and during early pregnancy for neural tube development), iron, and calcium, alongside routine advice to avoid alcohol entirely, avoid raw or undercooked meat, fish and eggs, avoid unpasteurized dairy and juice, and limit high-mercury fish. A prenatal vitamin is commonly recommended as a supplement to, not a replacement for, an otherwise balanced diet. None of this is a personalized nutrition plan; a doctor or a registered dietitian can account for individual health history in a way general guidance cannot.

Why this is an estimate, not a diagnosis

Every number this calculator produces, gestational age, trimester, due date and percent complete, is built from averages: an assumed 28-day cycle, ovulation around day 14, and a 280-day full term. Real pregnancies vary. Cycle length varies from person to person and month to month, ovulation does not always happen exactly on day 14, and only a small share of pregnancies actually deliver on the exact calculated due date. An early ultrasound, which measures the embryo or fetus directly, is generally considered more accurate than a calculation from the last period alone, and a doctor may adjust your dates based on it. Use this calculator as a quick, convenient estimate, and treat a doctor’s own dating as the number that actually matters for your care.

For a due date worked out the same way, the due date calculator is the more focused version of this same calculation, and working backward from a due date, or forward from ovulation, to estimate a conception window is covered by the conception calculator.

How a doctor decides between an LMP date and an ultrasound date is covered in pregnancy dating methods compared.