Pregnancy Calculator - Free Due Date Calculator & Pregnancy Week Calculator
Pregnancy Calculator
Typical: 21-35 days (28 average)
Enter your information and click "Calculate Due Date" to see your results
Understanding Pregnancy Calculators & Due Dates
What is a Pregnancy Calculator?
A pregnancy calculator (also called a due date calculator or pregnancy week calculator) is a tool that estimates your baby's due date based on various inputs like your last menstrual period (LMP), conception date, or ultrasound results.
Healthcare providers use a standard 40-week (280-day) pregnancy timeline from the first day of your last period. This is called gestational age and is the universal standard for tracking pregnancy, even though conception typically occurs about 2 weeks after your period starts.
How Does Due Date Calculation Work?
The most common method is Naegele's Rule: Add 280 days (40 weeks) to the first day of your last menstrual period. For women with regular 28-day cycles, this is highly accurate.
Adjustments: If your cycle is longer or shorter than 28 days, the calculator adjusts accordingly. For example, a 35-day cycle adds 7 days to your due date. Ultrasound dating in the first trimester is the most accurate method, typically within 3-5 days.
The Three Trimesters Explained
First Trimester (Weeks 1-13)
Critical organ development period. Highest risk of miscarriage (decreases significantly after week 12). Morning sickness typically peaks around week 9-10.
Second Trimester (Weeks 14-27)
Often called the "golden period." Energy returns, morning sickness subsides. Baby's movements felt (quickening) around weeks 16-25. Anatomy scan at week 20.
Third Trimester (Weeks 28-40+)
Final development and growth. Baby gains most weight. Increased prenatal visits. Full term begins at week 37. Most babies born between weeks 38-42.
Important Pregnancy Milestones
- •Week 6: Baby's heartbeat can be detected on ultrasound
- •Week 12: End of first trimester, miscarriage risk drops significantly
- •Week 20: Halfway mark! Anatomy scan performed
- •Week 24: Viability milestone - baby could survive with medical intervention
- •Week 37: Early full term - baby is considered fully developed
Frequently Asked Questions
How do I calculate my pregnancy due date?
To calculate your due date, enter the first day of your last menstrual period (LMP) and your average cycle length. The calculator uses Naegele's Rule, adding 280 days (40 weeks) to your LMP and adjusting for cycle length. You can also calculate from conception date (add 266 days) or ultrasound dating.
How accurate is a pregnancy due date calculator?
A pregnancy calculator provides an estimated due date with about 5% accuracy - only 5% of babies are born on their exact due date. Most babies arrive between 38-42 weeks. Ultrasound dating in the first trimester (especially at 8-12 weeks) is the most accurate method, typically within 3-5 days.
What is the difference between gestational age and fetal age?
Gestational age is calculated from the first day of your last menstrual period (LMP) and is the standard used by healthcare providers. Fetal age (conceptional age) is calculated from conception, typically 2 weeks less than gestational age since conception usually occurs about 14 days after LMP.
When does each trimester start and end?
The first trimester is weeks 1-13, the second trimester is weeks 14-27, and the third trimester is weeks 28-40+. The first trimester focuses on organ development, the second on growth and movement, and the third on final development and preparing for birth.
How does cycle length affect my due date calculation?
The standard pregnancy calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), your due date moves later. If shorter (e.g., 21 days), it moves earlier. Each day difference from 28 adjusts your due date by one day in the same direction.
Can I use this calculator if I did IVF or fertility treatments?
Yes! For IVF pregnancies, use the 'Conception Date' method and enter your embryo transfer date. For a 3-day transfer, add 3 days; for a 5-day transfer (blastocyst), add 5 days to your transfer date. This gives you the most accurate due date for IVF pregnancies.
What prenatal appointments should I expect during pregnancy?
Typical prenatal schedule: First visit at 8-10 weeks, monthly visits until 28 weeks, bi-weekly until 36 weeks, then weekly until delivery. Key tests include: NT scan (11-14 weeks), anatomy scan (18-22 weeks), glucose test (24-28 weeks), and Group B strep test (35-37 weeks).
What if my ultrasound due date differs from my LMP due date?
If the difference is 7+ days, your doctor will likely use the ultrasound date, especially from a first-trimester scan (most accurate). Early ultrasounds (8-12 weeks) are accurate within 3-5 days. LMP calculations can be off if you have irregular cycles, don't remember your exact LMP, or ovulated later than day 14.