pregnancy calculator by weeks pregnant

How this pregnancy calculator works

This calculator estimates gestational age using the first day of your last menstrual period (LMP), which is the same starting point many healthcare providers use. In obstetrics, pregnancy is counted from the LMP, not the day of conception. A full-term pregnancy is usually estimated at about 40 weeks (280 days) from LMP.

If your cycle is longer or shorter than 28 days, the calculator adjusts the estimate slightly. That can help provide a more realistic due date and week-by-week estimate.

How to count weeks pregnant

Week 1 starts before conception

It can feel strange, but week 1 begins on the first day of your period, before ovulation and conception occur. Most people conceive around week 2 or 3, depending on cycle length.

Gestational age vs fetal age

• Gestational age: Time measured from LMP (used in medical care).
• Fetal age: Time since conception (usually about 2 weeks less than gestational age).

Trimester breakdown by weeks

• First trimester: Week 1 through 13 weeks and 6 days.
• Second trimester: Week 14 through 27 weeks and 6 days.
• Third trimester: Week 28 to birth.

Tracking your pregnancy by week helps you anticipate appointments, screenings, and common symptoms.

Important week-by-week milestones

• 8 weeks: Early prenatal visit window for many patients.
• 10–13 weeks: First trimester blood tests and optional genetic screening.
• 18–22 weeks: Anatomy ultrasound scan.
• 24–28 weeks: Glucose screening for gestational diabetes.
• 28 weeks: Third trimester begins.
• 35–37 weeks: Group B strep testing in many practices.
• 37 weeks: Considered early term.
• 39–40 weeks: Full term and estimated due date window.

How accurate is a due date?

A due date is an estimate, not a guarantee. Many healthy pregnancies deliver within a range around the due date. Early ultrasound (especially first trimester) is often more accurate than LMP alone. If your provider updates your due date after an ultrasound, follow the clinical date they give you.

When to contact your healthcare provider

Call your provider urgently if you have heavy bleeding, severe abdominal pain, fever, persistent vomiting, severe headache, changes in vision, fluid leakage, or decreased fetal movement later in pregnancy. Always seek emergency care if you feel something is seriously wrong.

FAQ: pregnancy calculator by weeks pregnant

How many weeks pregnant am I if I know my due date?

You can count backward from the due date (40 weeks total) to estimate your current week. This calculator starts from LMP, which is the standard clinical approach.

What if I have irregular periods?

Irregular cycles can make LMP dating less precise. In that case, an ultrasound is usually the best method for confirming gestational age.

Can this replace medical advice?

No. This is an informational tool for planning and education. Your obstetrician, midwife, or healthcare team should guide your actual prenatal care.

Final thoughts

A pregnancy week calculator can help you understand your timeline, plan appointments, and follow developmental milestones. Use it as a helpful guide, and combine it with personalized care from your medical provider for the safest and most accurate pregnancy tracking.