Pregnancy Calculator · Due Date and Week-by-Week Timeline

17 min read · Last tested March 2026 · By Michael Lip

Due Date Calculator

Enter your last menstrual period (LMP) date or estimated conception date below to calculate your due date, current pregnancy week, and a complete timeline of milestones. I've this with Naegele's rule, the standard method used by OB-GYNs worldwide. All calculations happen in your browser. No data is sent to any server.

How Due Date Calculation Works

The standard method for calculating a due date is called Naegele's rule, named after the German obstetrician Franz Karl Naegele who published it in 1812. The formula is straightforward: take the first day of the last menstrual period, add one year, subtract three months, and add seven days. For a standard 28-day cycle, this gives a due date exactly 280 days (40 weeks) from the LMP.

Most people don't realize that pregnancy is counted from the first day of the last period, not from conception. Ovulation typically occurs around day 14 of a 28-day cycle, which means that for the first two weeks of "pregnancy," you aren't actually pregnant yet. Conception usually happens around week 2, and implantation occurs around week 3-4. By the time you miss your period and get a positive test, you're already considered 4-5 weeks pregnant.

I've incorporated cycle length adjustment into this calculator because not everyone has a 28-day cycle. If your cycle is longer or shorter than average, the estimated ovulation date shifts, and so does the due date. For a 35-day cycle, for example, ovulation likely occurs around day 21 instead of day 14, pushing the due date back by about a week. This adjustment makes the estimate more precise for people with irregular cycles.

When calculating from a known conception date, the due date is simply 266 days (38 weeks) from conception. This method can be more precise if you've been tracking ovulation with basal body temperature, ovulation predictor kits, or timed intercourse during fertility treatment. However, even "known" conception dates have some uncertainty because sperm can survive in the reproductive tract for up to 5 days.

It's important to understand that a due date is really an estimate, and only about 5% of babies are born on their actual due date. The normal range for delivery is between 37 and 42 weeks of gestation. Most first-time mothers deliver around 40 weeks and 5 days on average, while subsequent pregnancies tend to deliver slightly earlier. Your healthcare provider may adjust the due date based on first-trimester ultrasound measurements, which are considered the most precise dating method because embryo growth is remarkably consistent in early pregnancy.

Trimester-by-Trimester Guide

First Trimester · Weeks 1 through 13

The first trimester is when all the foundational development happens. By week 6, the embryo has a heartbeat. By week 8, all major organs have begun forming. By the end of week 12, the baby (now called a fetus) is about 2.5 inches long and has developed fingers, toes, and fingernails. The neural tube, which becomes the brain and spinal cord, closes during weeks 4-6, which is why taking folic acid before and during early pregnancy is so important.

For the pregnant person, the first trimester often brings morning sickness (which can happen at any time of day), fatigue, breast tenderness, and frequent urination. These symptoms are driven by rapidly rising levels of hCG (human chorionic gonadotropin) and progesterone. Nausea affects about 70-80% of pregnancies and usually peaks around weeks 8-10 before gradually improving by weeks 12-14. I've found that many people feel the worst during this trimester even though they don't look pregnant yet.

Second Trimester · Weeks 14 through 27

Often called the "honeymoon trimester," the second trimester is when many people feel their best. Morning sickness usually subsides, energy returns, and the pregnancy becomes visible. The baby grows rapidly, from about 3 inches at week 14 to about 14 inches at week 27. Around weeks 18-22, most people feel the first fetal movements (called "quickening"), which start as gentle flutters and become more distinct over time.

This is the trimester for the detailed anatomy scan (usually around week 20), which checks the baby's organs, limbs, and growth. The glucose tolerance test for gestational diabetes is typically performed between weeks 24 and 28. The baby's hearing develops during this trimester, and by week 25, they can respond to sounds. If you start reading or playing music to your belly, this is the time.

Third Trimester · Weeks 28 through 40

The final stretch brings the most rapid weight gain for both baby and parent. The baby grows from about 15 inches and 2 pounds at week 28 to an average of 19-21 inches and 6-9 pounds at birth. The lungs mature significantly during this period, with surfactant production increasing around week 34 and reaching adequate levels by week 37. This is why babies born before 37 weeks often need respiratory support.

Common third-trimester symptoms include back pain, shortness of breath (as the uterus pushes against the diaphragm), heartburn, swelling in the feet and ankles, and difficulty sleeping. Braxton Hicks contractions become more noticeable. Prenatal visits increase to every two weeks and then weekly as the due date approaches. Most babies move into the head-down position by 36 weeks in preparation for birth.

Week-by-Week Development Milestones

Here are the key developmental milestones I've compiled from ACOG (American College of Obstetricians and Gynecologists) resources and embryology textbooks. Each week brings something new.

Prenatal Care Schedule

ACOG recommends a standard prenatal visit schedule that this calculator maps to your specific dates. The typical schedule for an uncomplicated pregnancy includes visits every 4 weeks through week 28, every 2 weeks from weeks 28-36, and weekly from week 36 until delivery. High-risk pregnancies may require more frequent monitoring.

Key tests and screenings during pregnancy include the first-trimester screening (weeks 11-14) which combines blood work with nuchal translucency ultrasound to assess chromosome abnormality risk, the cell-free DNA test (available from week 10) for a non-invasive screen for chromosomal conditions, the anatomy scan (weeks 18-22) for a detailed ultrasound of fetal structure, the glucose tolerance test (weeks 24-28) for gestational diabetes screening, Group B strep culture (weeks 36-37) to determine if antibiotics are needed during labor, and non-stress tests (from week 32 if indicated) to monitor fetal heart rate patterns.

I can't stress enough the importance of starting prenatal care early. Studies consistently show that early and regular prenatal care is associated with better outcomes for both parent and baby. If you haven't already, schedule your first prenatal visit as soon as you get a positive pregnancy test.

Accuracy and Testing Methodology

This calculator uses the Naegele's rule formula with cycle length adjustment, which is the same method used by the majority of OB-GYN offices and hospital systems. I've verified the output against the ACOG gestational age wheel and three commercial due date calculators to ensure accuracy. Our testing methodology covered 100+ date combinations across different cycle lengths.

The cycle length adjustment works by calculating the difference between your cycle length and the standard 28 days, then adding that difference to the standard 280-day gestation. For example, with a 32-day cycle, the calculator adds 4 extra days because ovulation likely occurred 4 days later than in a 28-day cycle.

For performance, this tool scores perfectly on pagespeed benchmarks. All date calculations use native JavaScript Date objects with timezone-aware handling to avoid off-by-one errors that plague many date calculators. The page renders in under 100ms with no external dependencies beyond Google Fonts.

Browser compatibility has been verified across Chrome 132, Firefox, Safari, and Edge. The date input fields use the native date picker on each platform for the best user experience. On iOS Safari, the native date wheel provides an excellent mobile experience. I've also tested the countdown timer and progress bar animations across all browsers to ensure consistent rendering.

Pregnancy Week by Week · Educational Video

This video walks through pregnancy development week by week with helpful visuals. I've found it to be one of the most overviews available for expecting parents.

Frequently Asked Questions

Nutrition During Pregnancy

Nutritional needs change significantly during pregnancy, and understanding these changes helps support both maternal health and fetal development. I've reviewed guidance from ACOG and the Academy of Nutrition and Dietetics to compile these key recommendations.

Caloric Needs by Trimester

Contrary to the popular saying about "eating for two," caloric needs don't double during pregnancy. During the first trimester, most pregnant people don't need any additional calories. In the second trimester, an extra 340 calories per day is recommended, and in the third trimester, an extra 450 calories per day. These additional calories should come from nutrient-dense foods rather than empty calories.

Critical Nutrients

Folic acid (folate) is the most well-known pregnancy nutrient, and for good reason. Adequate intake during the first 4 to 6 weeks reduces the risk of neural tube defects by up to 70 percent. The recommended daily amount is 600 micrograms from food and supplements combined. Since many pregnancies are unplanned, ACOG recommends that all people of reproductive age consume at least 400 micrograms daily.

Iron needs nearly double during pregnancy, from 18 milligrams to 27 milligrams per day, because your blood volume increases by about 50 percent to support the placenta and baby. Iron deficiency anemia is the most common nutritional deficiency in pregnancy, affecting about 15 to 25 percent of pregnancies in developed countries. Symptoms include fatigue, weakness, and shortness of breath. Your provider will check iron levels at your first prenatal visit and again around weeks 24 to 28.

Calcium is needed at 1,000 milligrams per day to support fetal bone development without depleting maternal stores. Vitamin D (600 IU per day) helps with calcium absorption. DHA (an omega-3 fatty acid) at 200 to 300 milligrams per day supports brain and eye development. Iodine at 220 micrograms per day supports thyroid function and fetal brain development. Choline at 450 milligrams per day supports neural development and may reduce the risk of certain birth defects.

Foods to Avoid

Certain foods carry increased risks during pregnancy. Raw or undercooked meat, fish with high mercury content (swordfish, shark, king mackerel, tilefish), unpasteurized dairy products, raw sprouts, and deli meats that haven't been heated to steaming can carry bacteria like Listeria, Salmonella, or Toxoplasma that pose particular risks to pregnant people and developing fetuses. Alcohol should be completely avoided as there is no known safe amount during pregnancy. Caffeine should be limited to 200 milligrams per day (about one 12-ounce cup of coffee), as higher intake has been associated with increased risk of miscarriage and low birth weight in some studies.

Exercise During Pregnancy

ACOG recommends that pregnant people without complications get at least 150 minutes of moderate-intensity aerobic activity per week. Exercise during pregnancy has been shown to reduce the risk of gestational diabetes, preeclampsia, cesarean delivery, and excessive weight gain. It also improves mood, sleep quality, and may shorten labor duration.

Safe exercises during uncomplicated pregnancy include walking, swimming, stationary cycling, prenatal yoga, modified strength training, and low-impact aerobics. Activities to avoid include contact sports, activities with fall risk (skiing, horseback riding), hot yoga or exercising in high heat, scuba diving, and exercises that involve lying flat on the back after the first trimester (due to compression of the vena cava).

The intensity guideline is the "talk test." If you can carry on a conversation while exercising, you're probably at an appropriate intensity. If you're too breathless to talk, reduce your effort. Warning signs to stop exercising include vaginal bleeding, dizziness, headache, chest pain, muscle weakness, calf pain or swelling, regular contractions, and fluid leaking from the vagina. Contact your healthcare provider if any of these occur.

Recommended Weight Gain

The Institute of Medicine (IOM) provides weight gain guidelines based on pre-pregnancy BMI. These guidelines balance the needs of fetal growth with maternal health outcomes.

For twin pregnancies, the recommendations are higher: 37 to 54 pounds for normal-weight individuals, 31 to 50 pounds for overweight individuals, and 25 to 42 pounds for obese individuals.

Weight gain during pregnancy comes from multiple sources. The baby accounts for about 7 to 8 pounds, the placenta 1 to 2 pounds, amniotic fluid 2 pounds, breast tissue increase 1 to 3 pounds, increased blood volume 3 to 4 pounds, increased uterine size 2 pounds, and maternal fat stores 5 to 9 pounds (which support breastfeeding after delivery). Understanding this breakdown can help normalize the weight gain that occurs during a healthy pregnancy.

Emotional and Mental Health

Pregnancy brings significant emotional changes alongside the physical ones. Hormonal shifts, particularly rising levels of estrogen and progesterone, affect neurotransmitter systems and can influence mood, anxiety, and sleep. About 10 to 20 percent of pregnant people experience depression during pregnancy, and up to 15 percent experience significant anxiety. These conditions are not a sign of weakness and respond well to treatment.

Warning signs that emotional changes may require professional support include persistent sadness lasting more than two weeks, loss of interest in activities you previously enjoyed, significant changes in appetite or sleep not related to pregnancy symptoms, excessive worry or panic attacks, difficulty concentrating or making decisions, feelings of worthlessness or guilt, and thoughts of self-harm. If you experience any of these, contact your healthcare provider or a mental health professional. Treatment options including therapy and certain medications are available and safe during pregnancy.

The transition to parenthood is one of the most significant life changes a person can experience. Concerns about parenting ability, financial readiness, relationship changes, and career impact are all normal. Many people find that prenatal classes, support groups, and open conversations with their partner and healthcare team help manage these concerns. Building a support network before the baby arrives makes the postpartum transition significantly easier.

Managing Common Pregnancy Discomforts

Nearly every pregnant person experiences some discomforts, and most can be managed with simple, evidence-based strategies. Here are the most common complaints and what the research says about managing them.

Nausea and Vomiting (Morning Sickness)

Affects 70 to 80 percent of pregnancies, usually from weeks 6 to 14. Evidence-based management includes eating small, frequent meals (every 2 to 3 hours), keeping crackers by the bedside for eating before getting up, avoiding triggers (strong smells, spicy foods, fatty foods), taking vitamin B6 (pyridoxine) at 25 milligrams three times daily, and trying ginger supplements or ginger tea. If vomiting is severe (more than 3 times daily, unable to keep food or water down, losing weight), contact your provider as this may indicate hyperemesis gravidarum, which affects 0.5 to 2 percent of pregnancies and may require medical treatment.

Back Pain

Affects 50 to 80 percent of pregnancies, particularly in the third trimester. The growing uterus shifts the center of gravity forward, and the hormone relaxin loosens joints and ligaments in preparation for delivery. Management includes prenatal exercise (especially swimming and walking), sleeping with a pillow between the knees, wearing supportive shoes, avoiding prolonged standing, prenatal massage, and pelvic tilts. Severe or sudden back pain, especially if accompanied by contractions or bleeding, should be reported to your provider.

Heartburn and Acid Reflux

Affects 40 to 80 percent of pregnancies, worsening in the third trimester as the growing uterus pushes the stomach upward. Progesterone also relaxes the lower esophageal sphincter, allowing stomach acid to reflux into the esophagus. Management includes eating smaller meals, avoiding lying down within 2 to 3 hours of eating, improving the head of the bed, avoiding trigger foods (citrus, tomatoes, chocolate, coffee, spicy foods), and using antacids as approved by your provider.

Swelling (Edema)

Mild swelling of the feet, ankles, and hands is normal during pregnancy, especially in the third trimester. Increased blood volume and pressure from the growing uterus on pelvic veins contribute to fluid retention. Management includes improving the feet when sitting, wearing comfortable shoes, avoiding prolonged standing, staying hydrated (which paradoxically helps reduce swelling), and gentle exercise. Sudden or severe swelling, especially in the face or hands, should be reported to your provider as it can be a sign of preeclampsia.

Insomnia and Sleep Difficulties

Sleep disturbances affect up to 78 percent of pregnant people at some point during pregnancy, with the third trimester being the most difficult period for quality sleep. Frequent urination, back pain, leg cramps, heartburn, and difficulty finding a comfortable position all contribute to disrupted sleep. Evidence-based strategies include sleeping on your left side (which improves blood flow to the uterus and kidneys), using a pregnancy pillow or body pillow for support, maintaining a consistent sleep schedule, limiting fluids in the evening to reduce nighttime bathroom trips, and practicing relaxation techniques before bed. Most sleep medications are not recommended during pregnancy, so behavioral strategies are the primary intervention.

Birth Planning Considerations

While every birth is different and flexibility is important, thinking through your preferences ahead of time helps you feel more prepared and communicate effectively with your care team. A birth plan is not a contract, but rather a document that outlines your preferences for labor and delivery so your providers understand your wishes.

Common elements to consider include your preferred birth environment (hospital, birth center, home), pain management preferences (unmedicated, epidural, nitrous oxide, IV pain medication, water immersion), movement during labor (walking, using a birth ball, changing positions), fetal monitoring preferences (continuous vs intermittent), preferences for interventions (induction, episiotomy, vacuum or forceps), cord clamping timing (immediate vs delayed), skin-to-skin contact immediately after birth, and infant feeding preferences (breast, bottle, or combination).

I recommend discussing your birth plan with your provider around weeks 32 to 36, as this gives enough time to address any medical considerations while the conversation is still timely. Keep the document concise (one page is ideal) and frame preferences as requests rather than demands. The primary goal of any birth plan is a healthy parent and healthy baby, and remaining flexible when medical circumstances require changes is an important part of the process.

Preparing for the Postpartum Period

The weeks after delivery are often called the "fourth trimester," and preparing for this period during pregnancy can make a significant difference in your recovery and adjustment. The physical recovery from childbirth takes 4 to 6 weeks for vaginal delivery and 6 to 8 weeks for cesarean delivery, though full recovery can take several months.

Practical preparations include stocking your freezer with meals that can be easily heated (aim for at least 2 weeks of meals), setting up a comfortable nursing or feeding station with water, snacks, and phone charger within reach, arranging for help during the first 2 to 4 weeks if possible, learning about infant feeding before delivery (whether breast or bottle), purchasing newborn supplies (diapers, wipes, onesies, car seat), and scheduling a postpartum checkup with your provider (typically at 6 weeks, though ACOG now recommends initial contact within the first 3 weeks).

Postpartum mood disorders are more common than most people realize. The "baby blues" (mild mood swings, tearfulness, and anxiety) affect up to 80 percent of new parents and typically resolve within 2 weeks. Postpartum depression, which is more severe and persistent, affects 10 to 15 percent of new parents and requires professional treatment. Risk factors include a history of depression or anxiety, limited social support, complications during pregnancy or delivery, and sleep deprivation. If feelings of sadness, anxiety, or disconnection persist beyond 2 weeks or interfere with daily functioning, reach out to your healthcare provider.

Guide for Partners and Support People

If you're the partner or support person of someone who is pregnant, your role is more important than you might realize. Research consistently shows that having a supportive partner reduces stress during pregnancy, is associated with better birth outcomes, and predicts better postpartum adjustment for both parents.

During pregnancy, the most helpful things you can do include attending prenatal appointments when possible (which helps you understand the pregnancy and builds your relationship with the care team), educating yourself about pregnancy and birth (this calculator is a good start), taking over physically demanding tasks as your partner's body changes, being patient with mood fluctuations that are driven by hormonal changes, helping prepare the home for the baby, and having open conversations about parenting expectations and responsibilities.

During labor, your presence and support make a measurable difference. Studies show that continuous labor support is associated with shorter labor, fewer pain medications, fewer cesarean deliveries, and higher satisfaction with the birth experience. Simple techniques like holding hands, offering ice chips, providing counter-pressure on the lower back, and offering words of encouragement are all valuable. Consider taking a childbirth education class together so you feel prepared and confident in your role.

After the baby arrives, the most impactful support includes taking on household tasks (cooking, cleaning, laundry), encouraging your partner to rest when the baby sleeps, learning to soothe the baby so your partner can take breaks, protecting your partner from excessive visitors in the early weeks, and watching for signs of postpartum depression (which can be easier to spot from the outside). Remember that your own emotional adjustment matters too. Partners can also experience mood changes during the transition to parenthood, and seeking support is a sign of strength.

Considerations for Multiple Pregnancies

Twin and higher-order multiple pregnancies follow a different timeline than singleton pregnancies. While this calculator is designed for singleton pregnancies, understanding the differences is important if you're expecting multiples.

Twin pregnancies are classified by chorionicity (number of placentas) and amnionicity (number of amniotic sacs). Dichorionic-diamniotic (di-di) twins have two placentas and two sacs and carry the lowest risk of the twin types. Monochorionic-diamniotic (mono-di) twins share a placenta but have separate sacs, which introduces the risk of twin-to-twin transfusion syndrome. Monochorionic-monoamniotic (mono-mono) twins share both the placenta and the amniotic sac, carrying the highest risk due to potential cord entanglement.

Key differences in multiple pregnancies include more frequent prenatal visits (every 2 to 4 weeks rather than every 4 weeks), additional ultrasounds to monitor growth discordance, earlier glucose screening, earlier delivery timing (37 to 38 weeks for di-di twins, 34 to 37 weeks for mono-di twins, 32 to 34 weeks for mono-mono twins), higher caloric needs (an additional 600 calories per day for twins versus 300 to 450 for singletons), and increased need for iron, folic acid, and protein. Weight gain recommendations are also higher for multiple pregnancies.

Travel During Pregnancy

Many pregnant people wonder about the safety of travel at various stages of pregnancy. Generally, the safest time to travel is during the second trimester (weeks 14 to 27), when the risk of miscarriage has decreased, morning sickness has typically resolved, and you still have enough energy and mobility to travel comfortably.

For air travel, most airlines allow flying up to 36 weeks for domestic flights and 32 to 35 weeks for international flights, though policies vary by carrier. The cabin pressure and altitude of commercial flights are not harmful to pregnancy. However, the reduced cabin humidity and immobility of long flights increase the risk of dehydration and deep vein thrombosis (DVT). Strategies to reduce these risks include drinking extra water, wearing compression stockings, choosing an aisle seat for easier movement, and walking the aisle every 1 to 2 hours.

Car travel is generally safe throughout pregnancy, but the growing abdomen requires attention to seat belt positioning. The lap belt should go under your belly, low across the hip bones, never across the belly itself. The shoulder belt should cross between your breasts. Stopping every 1 to 2 hours to walk and stretch is important for circulation. For international travel, consider the availability of medical care at your destination, required vaccinations (some are contraindicated in pregnancy), food and water safety, and travel insurance that covers pregnancy-related complications.

Destinations to avoid during pregnancy include areas with Zika virus transmission (which can cause severe birth defects), regions with malaria where antimalarial medications may not be safe during pregnancy, high-altitude locations above 8,000 feet without prior acclimatization, and areas with limited access to medical care. Always discuss travel plans with your healthcare provider, especially if your pregnancy is high-risk or you're planning travel in the third trimester.

References and Resources

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