Pregnancy Due Date Calculator: How to Estimate When Your Baby Will Arrive

One of the first questions you will ask after a positive pregnancy test is: when is my baby due? It is a simple question with a surprisingly complicated answer. Due dates are estimates, not guarantees. Only about 5% of babies are born on their actual due date, and most full-term deliveries happen anywhere from 37 to 42 weeks.

Still, having an estimated due date is essential. It helps your healthcare provider track fetal development, schedule important tests and screenings, and plan for delivery. It also gives you a timeline to prepare your home, your work schedule, and your life for a new arrival.

Here is how due dates are calculated, what each trimester looks like, and what factors might shift your estimated date.

How Due Dates Are Calculated: Naegele's Rule

The standard method for estimating a due date has been used for over 200 years. It is called Naegele's rule, named after the German obstetrician who popularized it in the early 1800s.

The formula is straightforward:

This rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. Pregnancy is counted from the first day of your last period, not from conception, which typically happens about two weeks later. This means that during the first two weeks of your "pregnancy," you are not actually pregnant yet. It is a confusing convention, but it is how the medical world has standardized pregnancy dating.

Our Pregnancy Due Date Calculator uses this exact method and instantly shows your due date, current week, trimester, and key upcoming milestones.

Why Due Dates Are Estimates, Not Guarantees

The 280-day calculation assumes a perfectly regular 28-day cycle. But most women do not have textbook cycles. Normal cycle length ranges from 21 to 35 days, and even women with "regular" cycles can see their ovulation day shift by several days from month to month.

Here are the numbers on when babies actually arrive:

• 5% of babies are born on their due date

• 80% of babies are born within 2 weeks of their due date (38 to 42 weeks)

• 50% of first-time mothers deliver after their due date

• Average first pregnancy lasts about 41 weeks and 1 day

Think of your due date as the center of a window rather than a target. A healthy full-term birth can happen anywhere from 39 to 41 weeks (the period doctors consider "full term"), with 37 to 38 weeks classified as "early term" and 41 to 42 weeks as "late term."

Curious exactly how many days are between two dates? Our Date Difference Calculator can help you count the exact days between your LMP and today, or between any two dates you want to compare.

Trimester Breakdown: What to Expect

Pregnancy is divided into three trimesters, each lasting roughly 13 to 14 weeks. Here is what happens in each stage.

First Trimester (Weeks 1 through 12)

This is when the most dramatic development happens at the cellular level, even though you may not look pregnant at all.

Weeks 1 to 4: The fertilized egg implants in the uterine wall. The placenta begins forming. Most women do not know they are pregnant yet. A missed period at the end of this window is usually the first sign.

Weeks 5 to 8: The embryo's heart begins beating (around week 6). The brain, spinal cord, and major organs start developing. The embryo is about the size of a raspberry by week 8. Morning sickness often starts during this period, affecting roughly 70 to 80% of pregnant women.

Weeks 9 to 12: The embryo is now officially called a fetus. Fingers and toes form. By week 12, all major organs and body systems are in place and will continue maturing for the remaining months. The risk of miscarriage drops significantly after week 12. Many women begin sharing the news at this point.

Common symptoms: Nausea, fatigue, breast tenderness, frequent urination, food aversions, and mood changes. These are caused by surging hormones, especially hCG and progesterone.

Second Trimester (Weeks 13 through 27)

Often called the "golden trimester" because many of the unpleasant first-trimester symptoms fade while the baby is still small enough to be comfortable.

Weeks 13 to 16: Energy typically returns. The baby begins moving, though you may not feel it yet (especially in a first pregnancy). Sex can often be determined by ultrasound around week 16.

Weeks 17 to 20: Most women feel the baby move for the first time between weeks 18 and 22. This is called "quickening." The anatomy scan (a detailed ultrasound) is typically performed around week 20 to check all major organs and structures.

Weeks 21 to 27: The baby's hearing develops. They can respond to sounds and light. The baby practices breathing movements. By week 27, the baby is about 14.5 inches long and weighs approximately 2 pounds.

Common symptoms: Growing belly, back pain, round ligament pain, leg cramps, and the start of Braxton Hicks contractions (practice contractions that are irregular and usually painless). Calorie needs increase during this trimester. Use our Calorie Calculator as a general reference point, though pregnant women should follow their healthcare provider's specific nutrition guidance.

Third Trimester (Weeks 28 through 40+)

The final stretch. The baby is gaining weight rapidly and preparing for life outside the womb.

Weeks 28 to 32: The baby's lungs continue maturing. The brain develops rapidly. The baby weighs about 3 to 4 pounds by week 32 and is practicing sucking and swallowing. Babies born after 28 weeks have a greater than 90% survival rate with medical intervention, though they would need NICU care.

Weeks 33 to 36: The baby moves into a head-down position (in most cases) to prepare for birth. Space is getting tight, so movements may feel different, more rolling and stretching than kicking. The baby is gaining about half a pound per week.

Weeks 37 to 40+: The baby is considered "early term" at 37 weeks and "full term" at 39 weeks. The lungs are fully mature. Average birth weight is 6 to 9 pounds, and average length is 19 to 21 inches. If labor does not begin naturally by 41 to 42 weeks, your doctor will likely discuss induction.

Common symptoms: Shortness of breath, frequent urination (the baby is pressing on your bladder), difficulty sleeping, heartburn, swelling in feet and ankles, and increasingly strong Braxton Hicks contractions.

Wondering how far away your due date is? Use our Countdown Calculator to see exactly how many days remain until your big day.

Key Milestones by Week

Factors That Can Change Your Due Date

Several factors can cause your estimated due date to be adjusted during pregnancy.

Irregular menstrual cycles. If your cycles are longer or shorter than 28 days, Naegele's rule will not be perfectly accurate. Longer cycles mean ovulation happened later, so the due date should be pushed back. For example, a woman with a 35-day cycle likely ovulated around day 21 rather than day 14, shifting the due date by about a week.

First-trimester ultrasound dating. An ultrasound between weeks 8 and 12 can estimate the baby's age based on crown-to-rump length. This is the most accurate method of dating a pregnancy, typically within 3 to 5 days. If the ultrasound date differs from the LMP-calculated date by more than 7 days, your provider will usually adjust the due date to match the ultrasound.

IVF or assisted reproduction. If you conceived through IVF, your due date is calculated from the date of embryo transfer rather than LMP. For a day-5 blastocyst transfer, the due date is the transfer date plus 261 days (or minus 19 days to get the equivalent LMP, then use Naegele's rule). IVF due dates are among the most accurate because the exact conception timing is known.

Unknown last period. If you are unsure when your last period was (due to irregular cycles, breastfeeding, or coming off birth control), ultrasound dating becomes the primary method. First-trimester ultrasounds are more accurate for dating than second or third-trimester scans because early fetal growth rates are highly consistent across pregnancies.

Multiple pregnancies. Twins and higher-order multiples tend to arrive earlier. The average twin pregnancy delivers at 36 weeks, and triplets at around 32 weeks. Your provider will adjust expectations and monitoring accordingly.

When to See a Doctor During Pregnancy

Schedule your first prenatal appointment as soon as you have a positive pregnancy test. Most providers want to see you between weeks 6 and 10. After that, a typical prenatal visit schedule looks like this:

• Weeks 4 to 28: One visit every 4 weeks

• Weeks 28 to 36: One visit every 2 weeks

• Weeks 36 to delivery: One visit every week

Contact your provider immediately if you experience heavy bleeding, severe abdominal pain, sudden swelling of the face or hands, severe headaches with vision changes, or a significant decrease in fetal movement after week 28. These could indicate complications that require prompt evaluation.

Staying well-hydrated is important throughout pregnancy. Your blood volume increases by about 50% during pregnancy, and adequate hydration supports amniotic fluid levels. Use our Water Intake Calculator as a general starting point, but discuss your specific hydration needs with your provider, as recommendations increase during pregnancy.

Preparing for Your Due Date

While you cannot control exactly when your baby arrives, you can be ready within that delivery window.

Have your hospital bag packed by week 36. Include essentials for yourself (comfortable clothes, toiletries, phone charger, insurance documents), for the baby (going-home outfit, car seat installed in your vehicle), and for your support person (snacks, change of clothes, entertainment for what might be a long wait).

Finalize your birth plan by week 34. Discuss your preferences for pain management, delivery positions, and any specific requests with your provider. Be open to adjustments, as labor rarely follows a script.

Know the signs of labor. True labor contractions come at regular intervals, get closer together over time, and intensify rather than fading when you change positions. Other signs include your water breaking (a gush or slow trickle of fluid), a "bloody show" (mucus plug discharge), and persistent lower back pain that comes in waves.

Distinguish Braxton Hicks from true labor. Braxton Hicks contractions are irregular, do not get progressively stronger, and usually stop when you change activity or position. True labor contractions follow a pattern: they get longer, stronger, and closer together. When contractions are consistently 5 minutes apart, lasting 1 minute each, for at least 1 hour (the "5-1-1 rule"), it is time to call your provider or head to the hospital.

The Bottom Line

Your due date is the best estimate of when your baby will arrive, but it is just that: an estimate. Calculated using Naegele's rule (280 days from your last menstrual period) and refined by ultrasound, it gives you a target to plan around while acknowledging that babies operate on their own schedule.

Use our Pregnancy Due Date Calculator to find your estimated date and track which week and trimester you are in. Keep up with your prenatal appointments, ask your provider questions whenever they come up, and remember that whether your baby arrives at 38 weeks or 41 weeks, healthy full-term delivery is the goal. The date on the calendar matters far less than the care you receive along the way.