Calculate your gestational age, trimester, and due date from your last menstrual period or conception date — see prenatal milestones and screening windows aligned with ACOG guidelines.
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This pregnancy calculator accepts your last menstrual period start date (or estimated conception date) and calculates your current gestational age in weeks and days, your current trimester, and a timeline of key milestones — first heartbeat detection, end of first trimester, anatomy scan window, third trimester start, and estimated due date. It uses the standard obstetric convention of counting pregnancy from the first day of the LMP, which means gestational age is approximately two weeks ahead of fetal age. The calculator aligns with ACOG guidelines for gestational age calculation. If your periods are irregular or if an ultrasound has already established a different gestational age, use the ultrasound-derived date as your reference rather than this calculator's estimate.
Pregnancy is a term used to describe a woman's state over a time period (~9 months) during which one or more offspring develops inside of a woman. Childbirth usually occurs approximately 38 weeks after conception, or about 40 weeks after the last menstrual period. The World Health Organization defines a normal pregnancy term to last between 37 and 42 weeks. During a person's first OB-GYN visit, the doctor will usually provide an estimated date (based on a sonogram) at which the child will be born, or due date. Alternatively, the due date can also be estimated based on a person's last menstrual period.
While the due date can be estimated, the actual length of pregnancy depends on various factors, including age, length of previous pregnancies, and weight of the mother at birth. Studies have shown that fewer than 4% of births occur on the exact due date, 60% occur within a week of the due date, and almost 90% occur within two weeks of the due date. As such, while it is possible to be fairly confident that a person's child will be born within about two weeks of the due date, it is currently not possible to predict the exact day of birth with certainty.
Enter the first day of your last menstrual period (or your estimated conception date if you know it) and tap calculate. The result tells you your current gestational week and day — for example, "11 weeks, 3 days" — and which trimester you're in. The first trimester runs from week 1 through 13, the second from week 14 through 27, and the third from week 28 through 40. Milestones in your result (like "anatomy scan: weeks 18–22") are standard timing windows used by US OB providers; your provider may schedule these slightly differently based on your specific situation. If the calculator's gestational age differs from what your provider or ultrasound shows, always defer to the clinical assessment.
Gestational age is calculated from the first day of the last menstrual period:
Gestational Age (days)
Today's date − LMP date
Estimated Due Date (EDD)
LMP + 280 days (40 weeks)
Gestational Age (weeks) = Gestational Age (days) ÷ 7. The EDD is LMP + 280 days, consistent with ACOG's guidelines on gestational dating. If conception date is used instead of LMP, the calculator adds approximately 14 days to account for the standard LMP-to-ovulation interval before computing gestational age. A pregnancy at "8 weeks" by LMP means approximately 6 weeks of fetal development has occurred.
Most people expect "week 1 of pregnancy" to start at conception, but obstetric convention counts from the first day of your last period — roughly two weeks before ovulation and fertilization. That means when your provider says you're 6 weeks pregnant, the embryo has existed for only about 4 weeks. This matters because the first two weeks of gestational age, by the LMP method, technically precede conception. The convention was adopted because LMP is a known date while the exact day of ovulation and fertilization usually isn't. ACOG uses LMP dating as the default, with first-trimester ultrasound as the most accurate confirming method — when the two disagree by more than 5–7 days (depending on timing), the ultrasound date takes precedence for all subsequent scheduling. Understanding this distinction explains why your pregnancy "started" before you conceived, and why ultrasound dating often shifts the gestational age slightly from a pure LMP calculation.
Pregnancy unfolds in three trimesters, each with distinct developmental milestones your prenatal care will track:
The embryo implants, the neural tube closes (week 6), the heartbeat becomes detectable via transvaginal ultrasound (weeks 6–7), and all major organs begin forming. The risk of miscarriage is highest here — approximately 10–15% of confirmed pregnancies end in the first trimester, per ACOG. The first prenatal visit typically occurs around week 8–10 and may include dating ultrasound.
Often described as the most comfortable phase. Fetal movement (quickening) is typically felt between weeks 16–22. The anatomy scan — detailed ultrasound checking fetal structure and sex (if desired) — is scheduled between weeks 18–22. Gestational diabetes screening typically occurs around week 24–28.
Fetal growth accelerates rapidly. Group B strep testing happens around weeks 35–37. Fetal position matters increasingly; most babies settle head-down before week 36. The due date is week 40, but delivery between weeks 37 and 42 is considered normal term by ACOG standards.
When someone says they're "12 weeks," they're describing the current gestational week — they've completed 12 full weeks of pregnancy and are in the 13th week. "12 weeks and 3 days" is more precise and what this calculator returns. From a practical standpoint, hitting specific week thresholds matters because many announcements, genetic tests, and clinical decisions are tied to particular windows. The cell-free DNA (cfDNA) prenatal screening (also called NIPT — non-invasive prenatal testing) is typically offered starting at 10 weeks. The nuchal translucency ultrasound screening for Down syndrome is performed between weeks 11–14. Chorionic villus sampling (CVS), if opted for, is done between 10–13 weeks. First-trimester screening for chromosomal conditions closes at 13 weeks + 6 days — knowing your exact gestational age determines whether you're still within the testing window.
Pregnancy can be detected either by using pregnancy tests or by the woman herself noticing a number of symptoms, including a missed menstrual period, increased basal body temperature, fatigue, nausea, and increased frequency of urination.
Pregnancy tests involve the detection of hormones that serve as biomarkers for pregnancy and include clinical blood or urine tests that can detect pregnancy from six to eight days after fertilization. While clinical blood tests are more accurate, and can detect exact amounts of the hormone hCG (which is only present during pregnancy) earlier and in smaller quantities, they take more time to evaluate and are more expensive than home pregnancy urine tests.
Standard gestational dating assumes a 28-day menstrual cycle with ovulation at day 14. If your cycles are consistently longer (say, 35 days), ovulation likely occurs around day 21 rather than day 14, which means conception happened roughly a week later than the LMP calculation assumes — and the calculator would overestimate gestational age by about a week. If your cycles are shorter or highly irregular, the LMP method becomes less reliable and early ultrasound dating is especially important. ACOG recommends that in cases of uncertain LMP or irregular cycles, a first-trimester ultrasound crown-rump length (CRL) measurement provides the most accurate dating. If you input a conception date rather than LMP, the calculator bypasses cycle-length assumptions entirely and calculates gestational age directly from fertilization.
Taking certain medications during pregnancy can have lasting effects on the fetus. In the U.S., drugs are classified into categories A, B, C, D, and X by the FDA based on potential benefits vs. fetal risks. Consult your doctor regarding any medications you plan to use during pregnancy.
The Institute of Medicine recommends: 25-35 lbs for normal weight (BMI 18.5-24.9), 28-40 lbs for underweight (BMI < 18.5), 15-25 lbs for overweight (BMI 25-29.9), and 11-20 lbs for obese (BMI > 30).
Regular aerobic and strength-conditioning exercise are often recommended for pregnant women with uncomplicated pregnancies. The American College of Obstetricians and Gynecologists suggests that fetal injuries are unlikely from exercise. However, consult your doctor if you experience vaginal bleeding, shortness of breath, dizziness, headache, or decreased fetal movement.
Pregnancy requires different nutritional considerations due to increased energy and specific micronutrient requirements. Folic acid (Vitamin B9) helps decrease the risk of certain defects. DHA omega-3 is necessary for proper brain and retinal development and can only be obtained through the placenta during pregnancy or in breast milk after birth.
The most important input is an accurate LMP date — if you're unsure of the exact date, use the first day of the month your last period started as an approximation, understanding that error will propagate forward. Cycle length affects the accuracy of LMP-based dating significantly; women with cycles significantly longer or shorter than 28 days should confirm gestational age with an early ultrasound. Conception date, when known with confidence (e.g., from IVF transfer date or closely timed intercourse with ovulation testing), provides a more direct calculation. Ultrasound dating — particularly between 8–13 weeks using crown-rump length — is considered the most accurate clinical method and should override calculator estimates when there's a discrepancy.
Maya had her LMP on March 10, 2026. Today is June 27, 2026 — that's 109 days since LMP. Gestational age: 109 ÷ 7 = 15 weeks 4 days, solidly in the second trimester. Her anatomy scan window (weeks 18–22) is approximately 2.5–6 weeks away. She's just passed the first trimester and entered the typically more comfortable phase of pregnancy.
Brittany had a known conception date of May 15, 2026 (confirmed by ovulation testing). Today is June 27, 2026 — 43 days since conception. Gestational age using LMP convention (add ~14 days to conception): approximately 8 weeks 1 day, still in the first trimester. She's in the window for a first prenatal visit, dating ultrasound, and initial blood work. Her provider may order cfDNA screening in the next 2–3 weeks.
Write down the first day of your last period as soon as you suspect pregnancy — it's the most important date for every calculation that follows, and it's easy to forget within weeks.
If your cycles are irregular or you're uncertain of your LMP, request an early transvaginal ultrasound for accurate dating — LMP-based dates can be off by a week or more with irregular cycles.
Trimester boundaries (weeks 1–13, 14–27, 28–40) are guidelines, not sharp clinical cutoffs; your provider's scheduling is more important than the exact boundary date.
Keep a list of upcoming prenatal appointment windows derived from your gestational age so you can schedule proactively: first visit week 8–10, anatomy scan week 18–22, glucose screening week 24–28.
If your calculator date disagrees with your ultrasound date, always defer to the ultrasound — it's more accurate and your care team uses that date for all scheduling.
Remember that a full-term baby can arrive anywhere between 37 and 42 weeks; the 40-week due date is a midpoint estimate, not a deadline.
Enter the first day of your last menstrual period into the pregnancy calculator — it returns your current gestational age in weeks and days automatically. Gestational age counts from LMP, approximately two weeks before conception, per standard obstetric convention.
The first trimester is weeks 1–13, second trimester weeks 14–27, and third trimester weeks 28–40. Enter your LMP date to see exactly which trimester and week you're currently in.
Gestational age counts from LMP — typically 2 weeks before conception. Fetal age counts from fertilization. So a pregnancy at "8 weeks gestational age" reflects approximately 6 weeks of fetal development. Clinicians always use gestational age.
LMP-based calculation is a reliable estimate for women with regular 28-day cycles. For irregular cycles, accuracy can be off by a week or more, and early ultrasound dating — especially between 8–13 weeks — provides the most accurate gestational age. Always defer to ultrasound when dates conflict.
The second trimester begins at week 14 and runs through week 27. Most women find the second trimester more comfortable as morning sickness decreases and energy returns. The anatomy scan is scheduled in this window, typically at 18–22 weeks.
By 12 weeks, all major fetal organs have formed and the risk of miscarriage drops significantly. First-trimester genetic screening (including cfDNA/NIPT and nuchal translucency ultrasound) is either complete or closing. Many people choose this milestone to announce their pregnancy publicly.
Yes — if you know your conception date (from ovulation testing, IVF records, or closely timed intercourse), enter it directly and the calculator adjusts by adding approximately 14 days for the LMP convention to compute gestational age.
Brief disclaimer: This calculator provides educational gestational age estimates based on LMP or conception date using standard obstetric conventions. Results are for informational purposes only and do not constitute medical advice. Actual gestational age should be confirmed by a healthcare provider using clinical assessment and ultrasound dating. Always consult your OB-GYN or certified nurse-midwife for personalized prenatal care and pregnancy management.