How is my pregnancy week calculated?

Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. This means you're already considered 2 weeks pregnant when conception actually occurs. A full-term pregnancy is 40 weeks (280 days) from LMP. Your doctor may adjust the due date based on ultrasound measurements, especially in the first trimester.

How do I convert pregnancy weeks to months?

Pregnancy months don't divide evenly into 4-week blocks because most months have more than 28 days. The standard breakdown is: 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 23–26), Month 7 (Weeks 27–30), Month 8 (Weeks 31–35), Month 9 (Weeks 36–40). This is why pregnancy is often described as lasting 9 months but is actually 40 weeks.

How do I calculate pregnancy weeks with irregular periods?

If you have irregular periods, the LMP method may be less accurate. In this case, a first-trimester ultrasound (done between 6–13 weeks) is the most reliable way to determine your gestational age. You can also use the conception date method if you know when you conceived, or use the ultrasound method in this calculator by entering the gestational age measured during your scan.

How are pregnancy weeks calculated after IVF?

For IVF pregnancies, the calculation is based on the embryo transfer date and the age of the embryo. For a Day 5 blastocyst transfer, the due date is calculated as transfer date + 261 days. For a Day 3 embryo, it's transfer date + 263 days. This method is the most precise because the exact date of fertilization is known. Select the 'IVF' method in the calculator to use this approach.

How accurate is my due date?

Only about 5% of babies are born on their exact due date. A due date is an estimate — most babies arrive within a 5-week window (between 37 and 42 weeks). First-trimester ultrasound dating is accurate to within ±5–7 days. LMP-based dating can be off by 1–2 weeks, especially with irregular cycles. Your doctor may adjust your due date after early ultrasound measurements.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last menstrual period (LMP) — this is the standard used by doctors. Fetal age (also called embryonic or conceptional age) is counted from the date of conception, which is about 2 weeks after LMP. So at 10 weeks gestational age, your baby's fetal age is about 8 weeks. Medical professionals and this calculator use gestational age.

How accurate are the baby size comparisons?

The fruit and object size comparisons are based on medical averages and are meant to help you visualize your baby's growth. Every baby grows at a different rate, so actual sizes can vary. Your doctor tracks your baby's individual growth through ultrasound measurements. The length measurements switch from crown-to-rump (CRL) to crown-to-heel around week 20.

What are the three trimesters of pregnancy?

The First Trimester (Weeks 1–12) covers early development, organ formation, and is when morning sickness is most common. The Second Trimester (Weeks 13–26) is often called the 'golden period' — energy returns, the baby bump shows, and you feel first movements. The Third Trimester (Weeks 27–40) involves rapid baby growth, preparation for birth, and more physical discomfort as the due date approaches.

How do I know my conception date?

Most women don't know their exact conception date. If you know when you had intercourse during your fertile window, conception likely occurred within 24–48 hours after ovulation. For most women, ovulation happens about 14 days before the next period. If your cycle is 28 days, ovulation is around day 14. The calculator can estimate your conception date from your LMP or due date.

What if I don't remember my LMP date?

If you don't remember your last menstrual period date, you can use other methods: enter your known due date (if your doctor has already given you one), use the ultrasound method with your scan results, or enter your conception date if you know it. An early ultrasound remains the gold standard for dating when LMP is unknown.

Pregnancy Week Calculator

How Is Your Pregnancy Week Calculated?

Understanding the five methods for determining your current week

Pregnancy is typically dated from the first day of your last menstrual period (LMP), not from conception. This means you are already considered "2 weeks pregnant" at the time of conception. A full-term pregnancy is 40 weeks (280 days) from LMP.

1. Last Menstrual Period (LMP) — Naegele's Rule

The most common method. Add 280 days to the first day of your last period. If your cycle is not 28 days, the calculator adjusts automatically. Formula: EDD = LMP + 280 + (cycle length − 28). This is the standard method used by obstetricians.

2. Conception Date

If you know when conception occurred, this method adds 266 days (38 weeks) to that date. More precise because it skips the variable follicular phase of the menstrual cycle.

3. IVF Transfer Date

For IVF pregnancies, the embryo age at transfer is known precisely. A Day 5 (blastocyst) transfer adds 261 days; a Day 3 transfer adds 263 days. This is the most accurate method for IVF pregnancies.

4. Ultrasound Dating

An early ultrasound measures the embryo and reports a gestational age. The calculator uses this to back-calculate the LMP and project the due date. First-trimester ultrasounds are accurate to within ±5–7 days.

5. Known Due Date

If your doctor has already given you a due date, enter it directly. The calculator reverse-calculates your current gestational week, trimester, and all milestone dates from the due date.

Important: Only about 4–5% of babies are born on their exact due date. The EDD is the midpoint of a normal delivery range — most babies arrive between 37 weeks (early term) and 42 weeks (post-term).

Pregnancy Weeks to Months

How to convert pregnancy weeks into months and trimesters

Converting pregnancy weeks to months can be confusing because calendar months aren't exactly 4 weeks. Here's the standard obstetric breakdown:

First Trimester

Month 1 (Weeks 1–4) · Month 2 (Weeks 5–8) · Month 3 (Weeks 9–13). Major organs form, morning sickness begins, and the heartbeat is detectable by week 6.

Second Trimester

Month 4 (Weeks 14–17) · Month 5 (Weeks 18–22) · Month 6 (Weeks 23–26). The “golden trimester” — energy returns, you feel first movements, and the anatomy scan happens around week 20.

Third Trimester

Month 7 (Weeks 27–30) · Month 8 (Weeks 31–35) · Month 9 (Weeks 36–40). Rapid weight gain, lungs mature, and baby moves head-down in preparation for birth.

Important Assumptions & Limitations

Factors that can affect the accuracy of your pregnancy week calculation

Pregnancy week calculations provide an estimate, not a guarantee. Several factors affect accuracy:

Due date variance: Only about 5% of babies are born on their exact due date. Most arrive between 38–42 weeks.

Irregular cycles: If your periods are irregular, the LMP method may be less accurate. An early ultrasound provides a better estimate.

Baby size averages: Size comparisons are based on medical averages. Every baby grows at a different rate — your doctor tracks your baby's individual growth.

Ultrasound accuracy varies: First-trimester ultrasounds are accurate to ±5–7 days. Second-trimester scans are accurate to ±10–14 days.

Medical disclaimer: This calculator is for informational purposes only. Always consult your healthcare provider for medical advice about your pregnancy.

Common Mistakes to Avoid

Frequent errors that lead to incorrect pregnancy week calculations

Confusing gestational age with fetal age: Gestational age starts from LMP (2 weeks before conception). Fetal age starts from conception. Doctors use gestational age.

Using the wrong LMP date: Enter the first day your period started, not when it ended.

Ignoring cycle length: If your cycle is significantly shorter or longer than 28 days, adjust the cycle length for a more accurate result.

Expecting an exact due date: The "due date" is really a "due window" — full term spans from 37 to 42 weeks.

Frequently Asked Questions

Common questions about pregnancy weeks, gestational age, and calculation methods

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