Pregnancy Week Calculator - Calculate Your Pregnancy Week
Calculate what week of pregnancy you are in. Enter your last menstrual period, conception date, or due date to find your current week, trimester, estimated baby size, and development milestones.
Pregnancy Week Calculator
Find out what week of pregnancy you are in
Normal range 21-35 days
⚕️ Medical Disclaimer
This pregnancy week calculator is for informational and educational purposes only and should not replace professional medical advice, diagnosis, or treatment. Pregnancy dating provided by this tool is an estimate based on your reported last menstrual period and assumes a standard 28-day menstrual cycle. Actual gestational age should be confirmed by ultrasound with your healthcare provider, as individual variations in cycle length can affect accuracy. Every pregnancy is unique, and complications can arise at any stage regardless of calculator predictions. This tool cannot predict pregnancy outcomes or assess individual health risks. All pregnant individuals should seek regular prenatal care from qualified healthcare professionals throughout pregnancy. If you experience bleeding, severe abdominal pain, sudden swelling, severe headaches, or any concerning symptoms during pregnancy, contact your healthcare provider or emergency services immediately.
What is it?
Pregnancy is typically calculated in weeks and days, with an average pregnancy lasting 40 weeks (280 days) from the first day of the last menstrual period (LMP) to delivery. Understanding what week of pregnancy you are in is crucial for tracking fetal development, scheduling prenatal appointments, and preparing for labor. The pregnancy week calculator helps expectant parents and healthcare providers determine the current stage of pregnancy, estimate the due date, and understand what developmental milestones the baby has reached. Pregnancy is divided into three trimesters, each with distinct physical and emotional changes for the mother and significant development for the baby. The first trimester (weeks 1-13) focuses on organ formation and establishes the foundation for pregnancy. The second trimester (weeks 14-26) is characterized by rapid growth and the mother beginning to feel fetal movements. The third trimester (weeks 27-40) involves final weight gain and preparation for labor. Accurate pregnancy dating is essential for proper prenatal care, as it helps providers identify pregnancies at risk for complications, determine appropriate timing for interventions, and provide accurate due date predictions.
Formula Details
Pregnancy dating uses a standardized formula developed by medical professionals to ensure consistency in prenatal care across healthcare systems. The standard formula calculates pregnancy age as: Pregnancy Age (in weeks) = (Date Today - Last Menstrual Period Date) ÷ 7 days. For example, if the first day of your last menstrual period was January 1st and today is January 22nd, your pregnancy age would be (22 - 1) ÷ 7 = 3 weeks. Within each week, the day is calculated as: Day of Week = (Days Since LMP) mod 7. If you are on day 50 of pregnancy, you would be in week 8 (50 ÷ 7 = 7.14, rounded down to 7 weeks) and day 2 of that week (50 mod 7 = 1, so day 2). When using conception date, medical professionals add 14 days to the conception date to estimate the LMP, since ovulation and conception typically occur around day 14 of a 28-day cycle. The formula is: Estimated LMP = Conception Date - 14 days. When using a due date, the calculation is reversed: Estimated LMP = Due Date - 280 days, as the standard pregnancy duration is 280 days (40 weeks). These calculations assume a regular 28-day menstrual cycle. For those with longer or shorter cycles, adjustments can be made: Adjusted Pregnancy Age = Unadjusted Age + (Cycle Length - 28) ÷ 2. This adjustment accounts for the fact that cycle length differences primarily affect the timing of ovulation and LMP, not the total pregnancy duration from conception to delivery. For example, if you have a 35-day cycle, your ovulation occurs 7 days later, so you would add approximately 3.5 days to your calculated pregnancy age. Healthcare providers typically calculate all dates from the LMP for standardization, regardless of whether the actual conception date is known, because LMP provides a reliable external marker while exact conception date can be uncertain.
How to Calculate
The pregnancy week calculator offers three methods to determine your pregnancy stage. The most accurate method uses the first day of your last menstrual period (LMP), which is the standard medical reference point for calculating pregnancy age. If you know your LMP date, simply enter it into the calculator. The second method uses your conception date, which is typically about 14 days after the start of your menstrual cycle (around ovulation). If you know when conception occurred, enter that date and the calculator will work backward to establish your LMP. The third method uses your estimated due date, which is typically calculated as 280 days from your LMP. If you have been given a due date by your healthcare provider, enter it and the calculator will determine your current pregnancy week. Some calculators may also allow you to input your cycle length if it differs from the standard 28 days, which can make calculations more accurate for those with irregular cycles. After entering the relevant date, the calculator will display your current pregnancy week, the day of that week, total days pregnant, days remaining until your due date, your trimester, estimated baby size, key developmental milestones for the current week, and what to expect in terms of maternal changes.
Categories
| BMI Range | Category | Description |
|---|---|---|
Weeks 1 – 12 | First Trimester | Early pregnancy when all major organs begin forming. Morning sickness, fatigue, and breast tenderness are common symptoms during this stage. |
Weeks 13 – 26 | Second Trimester | The baby grows rapidly and many women feel their most comfortable during this period. Quickening (first fetal movements) typically occurs around weeks 18-22. |
Weeks 27 – 36 | Third Trimester | The baby gains weight and prepares for birth. Prenatal visits increase in frequency. Common symptoms include back pain, swelling, and fatigue. |
Weeks 37 – 38 | Early Full Term | The baby is nearly fully developed. Birth can occur safely at this stage, though optimal delivery is typically after week 39. |
Weeks 39 – 41+ | Full Term | The baby is fully developed and birth is expected around the due date. Only about 5% of babies are born exactly on their due date. |
Interpretation
Your pregnancy week provides important information about your baby's development stage, expected size, and what to anticipate in terms of physical symptoms and maternal changes. Weeks 1-4 represent a very early pregnancy where the fertilized egg travels to the uterus and implants. Weeks 5-8 involve rapid cell division and the beginning of organ formation, though most women do not yet show visible signs of pregnancy. Weeks 9-13 continue organ development with the baby becoming increasingly recognizable in ultrasound images. Around week 12, ultrasound can typically determine fetal viability and estimate due date more accurately. Weeks 14-20 mark the start of the second trimester when the baby grows rapidly and the mother typically begins feeling movements. The anatomy ultrasound around week 20 provides detailed information about fetal development and can determine gender. Weeks 21-26 involve continued rapid growth as the baby develops distinct sleeping and waking patterns. Weeks 27-30 mark the start of viability, meaning the baby could survive outside the womb with medical support if born prematurely. Weeks 31-36 involve significant weight gain for the baby and continued maternal physical changes. Weeks 37-40 represent full-term pregnancy, and the baby is considered term and ready for delivery at any time. Weeks 40+ represent post-term pregnancy, which requires medical monitoring. Understanding your pregnancy week helps you know what prenatal tests are recommended, when to expect certain physical changes, how your baby is developing, and when labor might be approaching. It also helps healthcare providers monitor the pregnancy appropriately and identify any deviations from normal progression that might require additional attention or intervention.
Limitations
Pregnancy week calculators use the Last Menstrual Period (LMP) method, which assumes a standard 28-day cycle with ovulation on day 14. Many women have longer, shorter, or irregular cycles, making LMP-based dating inaccurate by days or even weeks. First-trimester ultrasound (particularly between weeks 7-13) provides the most accurate dating and should be used to confirm or adjust the estimated due date. Only about 5% of babies arrive exactly on the calculated due date—the majority are born within 1-2 weeks of this estimate in either direction. This calculator does not account for individual risk factors, pregnancy complications, or medical conditions that may affect timing or care needs. Always rely on your healthcare provider's assessment for accurate pregnancy dating.
Health Risks
Pregnancy carries different risks at each stage. First trimester risks include miscarriage (most common in weeks 1-12, with risk declining significantly after week 12), ectopic pregnancy (implantation outside the uterus), and severe nausea/vomiting (hyperemesis gravidarum). Second trimester risks include early signs of preeclampsia, gestational diabetes (typically screened at weeks 24-28), and Rh incompatibility complications. Third trimester risks include preeclampsia (high blood pressure with organ damage), preterm birth, placental abruption, and in rare cases, stillbirth. Throughout pregnancy, general risks include gestational anemia, infections, blood clots (deep vein thrombosis), and depression. Regular prenatal care—including scheduled screenings, blood tests, and ultrasounds—dramatically reduces the likelihood of serious outcomes for both mother and baby.
Alternative Body Composition Measures
Beyond calendar-based pregnancy week calculators, several methods provide more accurate dating and monitoring. First-trimester ultrasound using crown-rump length measurement is the gold standard for dating—accuracy is within 5-7 days when performed before week 14. Second-trimester ultrasound uses fetal femur length, head circumference, and abdominal circumference to estimate gestational age. Blood tests measuring hCG levels (human chorionic gonadotropin) can confirm pregnancy and provide approximate dating. The Non-Stress Test (NST) and Biophysical Profile (BPP) assess fetal wellbeing in later pregnancy. Doppler ultrasound evaluates blood flow through the placenta and umbilical cord. Fetal movement counting (kick counts) is a simple monitoring method available to mothers from about week 28 onward.
Demographic Differences
Pregnancy risks and outcomes vary across demographic groups. Maternal age is one of the strongest risk factors: pregnancy complications are higher in women under 18 and over 35, with chromosomal abnormality risk (including Down syndrome) rising significantly after 35. First pregnancies (nulliparous) carry somewhat higher risks than subsequent pregnancies. BMI affects pregnancy outcomes—both underweight and overweight/obese women face elevated complication risks. Ethnicity influences certain outcomes: preterm birth rates are higher in some populations, as are rates of gestational diabetes and preeclampsia. Socioeconomic factors—including access to prenatal care, nutrition quality, and stress levels—significantly affect pregnancy outcomes. Smoking, alcohol, environmental exposures, and genetic factors all contribute to individual risk profiles. Smoking cessation during pregnancy is one of the single most impactful steps a pregnant woman can take.
Tips
- Schedule your first prenatal appointment as soon as you confirm pregnancy — early prenatal care is crucial for both mother and baby
- Start taking a prenatal vitamin with folic acid immediately, even before your first doctor's visit, to support early neural tube development
- Keep a pregnancy journal to track symptoms, note questions for your doctor, and record milestones
- Write down questions between appointments and bring them to each prenatal visit — appointments pass quickly
- Learn the warning signs specific to each trimester so you know when to seek immediate medical attention
- Stay hydrated and eat a balanced diet rich in iron, folic acid, calcium, and protein throughout all trimesters
- Gentle, pregnancy-safe exercise like walking, swimming, or prenatal yoga is beneficial unless your doctor advises otherwise
- Prepare your birth plan and hospital bag by week 35-36 so you are ready when the time comes
Frequently Asked Questions
How accurate is pregnancy dating?
Pregnancy dating from the first day of the last menstrual period (LMP) is accurate within 3-5 days if you know your LMP date and have regular 28-day cycles. If your menstrual cycles are irregular, dating is less accurate. An ultrasound performed in the first trimester (especially before week 13) can establish pregnancy dating accurate within 3-5 days and is more reliable than dating after the first trimester. Second trimester ultrasound can date pregnancy within 1-2 weeks, and third trimester ultrasound within 2-4 weeks, so early dating is more accurate overall.
When should I schedule my first prenatal visit?
Most healthcare providers recommend scheduling the first prenatal visit when you have missed a period and have a positive pregnancy test, typically around week 8-10 of pregnancy. Some providers may ask you to schedule earlier if you have risk factors or medical conditions. Early prenatal care helps establish accurate dating, identify any risk factors, and begin appropriate screening and monitoring.
What if my due date seems wrong?
If you are confident about when your last menstrual period was or when conception occurred, and your due date from the healthcare provider seems significantly different (more than a week), an early ultrasound can help establish more accurate dating. Ultrasound dating is most accurate in the first trimester, so if timing allows, this ultrasound should be done as early as possible. If you are already in the second or third trimester, updating the due date based on early ultrasound dating is still preferable if the ultrasound is significantly different from LMP-based dating.
Can I deliver before my due date?
Yes, labor can occur before the estimated due date. Delivery before 37 weeks is considered premature, though some modern medicine considers 37 weeks early term. Delivery between 37-39 weeks is early term. Delivery at 39+ weeks is at term. Most pregnancies deliver within two weeks before or after the due date, but any time from week 37 onward is considered safe for delivery. Delivery before week 37 requires medical evaluation and is treated more cautiously to manage risks associated with prematurity.
What is the difference between trimesters?
The three trimesters represent distinct phases of pregnancy. The first trimester (weeks 1-13) involves rapid organ development in an invisible pregnancy with common symptoms like morning sickness. The second trimester (weeks 14-26) involves rapid fetal growth and is often called the "honeymoon period" with increased energy and the mother feeling fetal movements. The third trimester (weeks 27-40) involves final preparation for birth with significant weight gain and maternal discomfort. Each trimester has different prenatal screening and testing recommendations.
How is baby size estimated?
Baby size is estimated through ultrasound measurements. The measurements taken include biparietal diameter (head width), femur length (thighbone), and head circumference. These measurements are compared to expected values for the gestational age. Estimated fetal weight is calculated from these measurements, though ultrasound estimates of weight can have a margin of error of 10-15%, especially as pregnancy advances. Factors like genetics and maternal nutrition influence baby size, so there is significant normal variation among babies at the same gestational age.
What screening tests are recommended during pregnancy?
Recommended screening tests vary but typically include first trimester screening (weeks 11-14) with nuchal translucency ultrasound and blood tests to assess risk for chromosomal abnormalities. Second trimester screening includes the anatomy ultrasound (weeks 18-22) and optional quad screen blood test. Gestational diabetes screening is performed around week 24 with a glucose tolerance test. Group B Streptococcus (GBS) testing is done around week 35-37. Some women opt for non-invasive prenatal testing (NIPT) or cell-free fetal DNA screening, which can be done as early as week 9-10. Discuss with your healthcare provider which screening tests are appropriate for your situation.
What should I do if I notice decreased fetal movement?
Starting around week 24-28, you should familiarize yourself with your baby's normal movement patterns. If you notice a significant decrease in movement, especially in the third trimester, contact your healthcare provider immediately. A reduction in fetal movement can sometimes indicate a problem requiring evaluation, including fetal monitoring. Your healthcare provider can perform a non-stress test or other monitoring to ensure the baby is doing well. Never assume decreased movement is normal without medical evaluation.
Can I travel while pregnant?
Travel during pregnancy is generally safe during the second trimester (weeks 14-20) when morning sickness has usually resolved, and you are not yet heavily pregnant. Travel is not recommended in the first trimester (due to miscarriage risk and morning sickness) or after week 36 (due to risk of labor). Air travel is generally considered safe until week 36, though airlines often have restrictions around week 34-36. Long car trips should include frequent stops to walk and stretch. Discuss travel plans with your healthcare provider to ensure they are appropriate for your individual pregnancy.
What is normal weight gain during pregnancy?
Recommended weight gain during pregnancy varies based on pre-pregnancy weight: women with normal pre-pregnancy weight (BMI 18.5-24.9) should gain 25-35 pounds; underweight women (BMI < 18.5) should gain 28-40 pounds; overweight women (BMI 25-29.9) should gain 15-25 pounds; and obese women (BMI ≥ 30) should gain 11-20 pounds. Weight gain is not linear, with most women gaining little in the first trimester and more in the second and third trimesters. Discuss appropriate weight gain targets with your healthcare provider based on your individual circumstances.
When should I stop working during pregnancy?
The timing to stop working during pregnancy varies based on job type, physical demands, work environment, maternal health, and pregnancy complications. Many women work throughout pregnancy without problems, especially in less physically demanding jobs. Jobs involving exposure to harmful substances, significant physical labor, prolonged standing, or jobs with high stress may warrant earlier cessation of work. Your healthcare provider and employer should discuss accommodations and when stopping work would be medically advisable. Many women take maternity leave starting around week 37-39 to prepare for labor.
Is it normal to have spotting or bleeding during pregnancy?
Light spotting in early pregnancy can occur around the time of implantation (usually before a positive pregnancy test) and is often normal. Light spotting can also occur after sexual intercourse or cervical exams. However, any vaginal bleeding during pregnancy should be reported to your healthcare provider, especially if accompanied by abdominal pain or cramping. Significant bleeding can indicate complications such as miscarriage, ectopic pregnancy, placental abnormalities, or infection. Any concerning bleeding warrants prompt medical evaluation.
References & Sources
- [1]American College of Obstetricians and Gynecologists (ACOG). Medically Indicated Late-Preterm and Early-Term Deliveries. Practice Bulletin. 2021.
- [2]Robinson HP, Fleming JE. Accuracy of ultrasound dates in early pregnancy. Aust NZ J Obstet Gynaecol. 1975;15(3):89-92.
- [3]World Health Organization. Recommendations on Antenatal Care for a Positive Pregnancy Experience. 2016.
- [4]National Institute of Child Health and Human Development. Pregnancy and Childbirth: What to Expect. 2020.
- [5]March of Dimes. Understanding Pregnancy Weeks and Trimesters. 2022.
These references are provided for educational purposes. Always consult healthcare professionals for medical advice.