pregnancy calculator week by week

How this pregnancy calculator works

This calculator uses the same basic medical model used in most prenatal clinics: pregnancy is dated from the first day of your last menstrual period, not from conception. A full-term pregnancy is typically estimated at 40 weeks (280 days) from LMP for a 28-day cycle.

If your cycle is longer or shorter than 28 days, the calculator adjusts your estimated due date accordingly. That makes it more useful than a one-size-fits-all due date tool.

• LMP-based dating: Standard clinical method for early pregnancy estimation.
• Cycle-length adjustment: Helps improve accuracy when cycles differ from 28 days.
• Week-by-week timeline: Shows date ranges and milestones from week 1 to week 40.

What your results mean

Gestational age

Gestational age is shown in completed weeks and extra days (for example, 18 weeks and 3 days). This is the format used in prenatal appointments, ultrasounds, and most pregnancy resources.

Estimated due date

Your due date is an estimate, not an exact prediction. Many healthy pregnancies deliver between 37 and 42 weeks. It is normal for babies to arrive before or after the calculated due date.

Trimester

Pregnancy is divided into three trimesters:

• First trimester: Weeks 1–13
• Second trimester: Weeks 14–27
• Third trimester: Weeks 28–40+

Pregnancy week by week: quick overview

First trimester (weeks 1 to 13)

Early development happens fast. The neural tube forms, the heart starts beating, and major organs begin to develop. Common symptoms include fatigue, nausea, breast tenderness, and frequent urination.

• Week 4–5: Implantation and rising hCG
• Week 6–8: Early heart activity and rapid organ formation
• Week 10–12: Embryo becomes fetus, movement begins

Second trimester (weeks 14 to 27)

This is often considered the most comfortable phase. Energy may improve, and many parents begin to feel fetal movement. Anatomy scans are commonly performed around 18–22 weeks.

• Week 14–16: Facial features and limbs become more defined
• Week 18–22: Detailed anatomy ultrasound window
• Week 24–27: Lungs and brain continue significant growth

Third trimester (weeks 28 to birth)

During the final trimester, weight gain and organ maturation accelerate. Prenatal visits usually become more frequent as you approach labor.

• Week 28–32: Brain, lungs, and fat stores develop further
• Week 33–36: Baby often turns head-down
• Week 37–40: Full-term window and labor preparation

Weeks to months pregnancy chart (approximate)

• Month 1: Weeks 1–4
• Month 2: Weeks 5–8
• Month 3: Weeks 9–13
• Month 4: Weeks 14–17
• Month 5: Weeks 18–22
• Month 6: Weeks 23–27
• Month 7: Weeks 28–31
• Month 8: Weeks 32–35
• Month 9: Weeks 36–40

Tips to improve date accuracy

• Use the exact first day of your last period if known.
• Enter your real average cycle length, not an estimate from memory if possible.
• Compare calculator results with early ultrasound dating for the most reliable timeline.
• If cycles are irregular, discuss due date interpretation with your prenatal provider.

Frequently asked questions

Can this calculator tell me my exact delivery day?

No. It gives a best estimate. Only a small percentage of births occur exactly on the due date.

What if my ultrasound date is different?

Early ultrasound can be more accurate than LMP dating in some cases. Your clinician may update your estimated due date based on scan findings.

Can I use this if I conceived via IVF?

IVF pregnancies are often dated using embryo transfer and embryo age rather than LMP alone. Ask your fertility clinic or OB provider for exact medical dating.