pregnancy due date calculator week by week

How this pregnancy due date calculator week by week works

This calculator estimates your due date using standard obstetric methods. If you enter your LMP, it uses a 280-day pregnancy model and adjusts for cycle length. If you enter conception date, it adds 266 days.

After calculation, you get a practical summary:

• Your estimated due date
• Your estimated gestational age today (weeks + days)
• Your trimester
• A short week-by-week outlook for the upcoming weeks

How doctors estimate your due date

1) Last Menstrual Period (LMP)

This is the most common method. Pregnancy is dated from the first day of your last period, not from ovulation. Standard due date formula: LMP + 280 days.

If your cycle is longer or shorter than 28 days, estimated ovulation shifts. This calculator adjusts your due date by adding or subtracting the cycle difference.

2) Conception date

If you know conception fairly accurately, due date is estimated as conception + 266 days. This is often useful when tracking ovulation closely.

3) Ultrasound dating

In clinical care, early ultrasound can refine dating, especially if periods are irregular or LMP is uncertain. Your clinician may adjust your estimated due date based on fetal measurements.

Week-by-week pregnancy milestones (quick guide)

Weeks 1-4

• Pregnancy dating begins from LMP.
• Fertilization and implantation happen near the end of this window.

Weeks 5-8

• Early organ development starts.
• Common symptoms: fatigue, nausea, breast tenderness.

Weeks 9-12

• First trimester progresses quickly.
• Many people begin prenatal visits and basic bloodwork.

Weeks 13-16

• Second trimester begins.
• Energy often improves.

Weeks 17-20

• Some feel first fetal movement ("quickening").
• Anatomy ultrasound is commonly performed around this phase.

Weeks 21-28

• Growth accelerates.
• Glucose screening usually occurs between 24-28 weeks.

Weeks 29-36

• Third trimester development intensifies.
• Breathing practice movements and brain growth continue.

Weeks 37-40

• Pregnancy is considered full term from 39 weeks.
• Labor may begin anytime in this range.

Important reminders about due dates

• Due dates are estimates, not exact predictions.
• Only a small percentage of births happen on the exact due date.
• Deliveries commonly occur between 37 and 42 weeks.

Frequently asked questions

How accurate is a due date calculator?

It is usually a good estimate. Accuracy improves when dates are known and cycles are regular. Early ultrasound can improve dating further.

Can my due date change?

Yes. If early ultrasound suggests a different gestational age, your provider may adjust your due date.

What if my cycles are irregular?

LMP-based estimates may be less precise. Ultrasound dating is especially useful in this situation.

Is this calculator for twins?

It can still estimate gestational age and a due date, but twin pregnancies are managed differently. Follow your OB/GYN or midwife for personalized timing and monitoring.

Medical disclaimer: This tool is for educational use and does not diagnose, treat, or replace professional medical care. Always confirm pregnancy timing and prenatal plans with your healthcare provider.