How do I calculate my due date from my last period?

Add 40 weeks (or 280 days) to the first day of your last menstrual period. This gives you an estimated due date based on a typical 28-day cycle. For a quick calculation, use Lively's pregnancy due date calculator above. Remember to confirm with your healthcare provider.

Can I calculate my due date if I have irregular periods?

If you have irregular periods, calculating due dates from your last period is less accurate. Your doctor will likely use an early ultrasound to measure your baby and determine a more precise due date. The ultrasound method is the most reliable for people with irregular cycles.

What if I don't remember my last period date?

If you don't remember when your last period started, don't worry. Your healthcare provider can estimate your due date using an ultrasound, which measures your baby's size. This is often more accurate anyway, especially in early pregnancy. Start tracking your cycles with Lively for future reference.

How accurate are due date calculators?

Due date calculators provide helpful estimates but aren't 100% accurate. They assume a 28-day cycle and ovulation on day 14, which doesn't apply to everyone. Ultrasound dating, especially in the first trimester, is the most accurate method. Only about 5% of babies are born on their exact due date.

Can my due date change during pregnancy?

Your estimated due date may be adjusted after your first ultrasound if the measurements suggest a different gestational age. However, once confirmed by ultrasound, your due date typically won't change unless there's a clear reason. Even so, remember that babies rarely arrive exactly on schedule.

What's the difference between gestational age and fetal age?

Gestational age is measured from the first day of your last period (what doctors use), while fetal age is measured from conception, which is typically about 2 weeks later. When someone says you're "8 weeks pregnant," they're referring to gestational age, your baby's actual age is closer to 6 weeks.

When should I take a pregnancy test?

For the most accurate results, take a pregnancy test on the first day of your missed period or a few days after. Testing too early can result in a false negative because hCG levels haven't built up enough yet. Use Lively's pregnancy test calculator to find your optimal testing window.

How far along am I in my pregnancy?

To calculate how far along you are, count the number of weeks from the first day of your last period. For example, if your last period started 6 weeks ago, you're 6 weeks pregnant. Use Lively's pregnancy calculator or app to track your progress week by week with personalized insights.

Free Due Date Calculator | Calculate Your Due Date

How is my due date calculated?

Pregnancy due dates are calculated by adding 40 weeks (280 days) to the first day of your last menstrual period (LMP). This method is used because pinpointing the exact conception date is difficult, sperm can survive up to 5 days, and ovulation timing varies.

Here's how it works

1Start with the first day of your last period
2Add 40 weeks (280 days)
3That's your estimated due date!

Example: If your last period started on January 1, your estimated due date would be around October 8.

This calculation assumes a typical 28-day cycle with ovulation around day 14. If your cycle is shorter or longer than 28 days, your actual due date may differ. That's why an ultrasound confirmation with your doctor is essential.

When will my baby arrive? Quick reference

Wondering what month you might be due? Here's a quick guide based on conception month (assuming a 40-week pregnancy):

Conceived in January → Due in October

Conceived in February → Due in November

Conceived in March → Due in December

Conceived in April → Due in January

Conceived in May → Due in February

Conceived in June → Due in March

Conceived in July → Due in April

Conceived in August → Due in May

Conceived in September → Due in June

Conceived in October → Due in July

Conceived in November → Due in August

Conceived in December → Due in September

Remember, very few babies actually arrive on their due date! It's completely normal to give birth anywhere between 37 and 42 weeks of pregnancy.

How accurate is a pregnancy due date?

Here's the truth: Due dates are educated estimates, not guarantees. Only about 5% of babies arrive exactly on their due date. Most babies are born between 37 and 42 weeks of pregnancy, that's a 5-week window!

Think of your due date as a target month rather than a specific day. The Lively app helps you track your pregnancy week by week, so you'll know what to expect no matter when your baby decides to arrive.

Why your due date matters for prenatal care

While your due date is an estimate, it plays an important role in your prenatal care. Your healthcare provider uses it to:

Schedule important tests and screenings at the right time
Monitor your baby's growth and development appropriately
Determine if your pregnancy is full-term, preterm, or post-term
Plan timing for induction if needed
Track milestones throughout your pregnancy journey

Knowing your gestational age helps ensure you and your baby get the right care at the right time. Use the Lively app to follow your baby's development week by week and get personalized tips for each stage of pregnancy.

Free calculators

Due date calculator

How is my due date calculated?

Here's how it works

When will my baby arrive? Quick reference

How accurate is a pregnancy due date?

Why your due date matters for prenatal care

Track your pregnancy week by week

Frequently asked questions

Important notice