Calculate Pregnancy Week, Due Date, and Trimester
This tool is for education only and does not replace medical advice. Always confirm dating and due date with your healthcare provider.
What is a pregnancy weeks calculator?
A pregnancy weeks calculator estimates how far along a pregnancy is, based on a date you provide. Most calculators use the first day of your last menstrual period (LMP), because that is the standard method used in obstetrics. From that date, the tool estimates gestational age in weeks and days, your estimated due date, your current trimester, and important milestone dates.
If your cycle is longer or shorter than 28 days, the estimate can shift slightly. That is why this calculator includes cycle length. It gives you a closer starting estimate before your provider confirms dates with ultrasound and clinical assessment.
How to use this calculator
Step-by-step
- Enter the first day of your last menstrual period (LMP).
- Enter your average cycle length in days (28 is prefilled).
- Choose the date you want to calculate for (today is set automatically).
- Click Calculate to view your week, trimester, and due date estimate.
You can also use the “as of date” field to plan ahead. For example, if you want to know how far along you will be on a holiday, travel date, or appointment date, just set that future date and recalculate.
Understanding your result
Gestational age (weeks + days)
Pregnancy is usually described in weeks and days. For example, 18 weeks and 4 days means you are in week 19 of pregnancy, but have completed 18 full weeks so far.
Estimated due date (EDD)
Most due dates are calculated as 280 days (40 weeks) from LMP. If cycle length differs from 28 days, the estimate adjusts by the difference. This gives a more personalized estimate, though early ultrasound can still be more precise for some people.
Trimester
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: weeks 28–40+
Pregnancy milestones by trimester
First trimester (weeks 1–13)
- Fertilization and implantation occur early.
- Major organs begin forming.
- Common symptoms include nausea, fatigue, and breast tenderness.
- First prenatal visit and early blood work are typically scheduled.
Second trimester (weeks 14–27)
- Energy often improves for many people.
- Anatomy ultrasound is commonly done around week 18–22.
- Baby movement may become noticeable.
- Routine screening and blood pressure checks continue.
Third trimester (weeks 28–40+)
- Baby gains weight rapidly and positions for birth.
- Appointments usually become more frequent.
- Braxton Hicks contractions may appear.
- Labor planning and birth preparation become the focus.
Why your due date may change
It is common for due dates to be adjusted, especially after an early ultrasound. If ovulation happened earlier or later than expected, LMP-based dating can be off by several days. Ultrasound measurements in early pregnancy can provide a more accurate estimate for many pregnancies, and your care team may update your EDD based on those findings.
Frequently asked questions
Is pregnancy exactly 9 months?
Not exactly. Clinically, pregnancy is tracked as 40 weeks from LMP, which is slightly longer than 9 calendar months in many cases.
Can irregular cycles affect this calculator?
Yes. If cycles are irregular, LMP-based estimates are less precise. This calculator still gives a useful estimate, but ultrasound-based dating is especially important in that situation.
What if I conceived through IVF?
IVF pregnancies are often dated using embryo transfer date and embryo age rather than LMP alone. Ask your fertility clinic or obstetric provider for the exact dating method they recommend.
When to call your healthcare provider
Always seek medical advice if you have concerns. Contact your provider urgently for symptoms such as:
- Heavy bleeding or severe abdominal pain
- Persistent severe headache, vision changes, or swelling
- Fever, chills, or signs of infection
- Severe vomiting with inability to keep fluids down
- Reduced fetal movement later in pregnancy
Bottom line
A pregnancy weeks calculator is a practical way to estimate how far along you are and understand your timeline. It helps with planning appointments, tracking milestones, and setting expectations. Use it as a guide, and rely on your care team for official dating and individualized prenatal care.