Is due date based on 38 or 40 weeks?

40 weeks from LMP (last menstrual period). Actual pregnancy is 38 weeks from conception, but doctors add 2 weeks to account for pre-ovulation time.

Why do doctors add 2 weeks on to pregnancy?

Pregnancy is counted from first day of your last period, not conception. Since ovulation occurs ~2 weeks after LMP, those 2 weeks are included even though you weren't pregnant yet.

Is EDD 39 or 40 weeks?

EDD (Estimated Due Date) is calculated at 40 weeks from LMP. However, full term is 39-40 weeks, and only 5% of babies are born on their exact due date.

Is due date based on ovulation date?

Can be calculated from ovulation by adding 266 days (38 weeks). More accurate than LMP for irregular cycles or known conception date.

Can my due date change?

Yes, especially after first trimester ultrasound if measurements differ >1 week from LMP calculation. Early ultrasounds are most accurate for dating.

How accurate is your due date by ultrasound?

First trimester ultrasound is most accurate (±5-7 days). Accuracy decreases later: second trimester (±2 weeks), third trimester (±3 weeks).

How am I 4 weeks pregnant if I conceived 2 weeks ago?

Pregnancy is dated from LMP, not conception. At conception (2 weeks after LMP), you're already considered "2 weeks pregnant."

Is 3 weeks pregnant actually 3 weeks?

No, at "3 weeks pregnant" the embryo is only 1 week old. Gestational age (from LMP) is 2 weeks ahead of actual fetal age.

Why are 40 weeks counted as 9 months?

40 weeks = 280 days = 10 lunar months (28 days each) or 9.2 calendar months. Pregnancy is actually slightly longer than 9 months.

What is the 40 week rule?

Standard pregnancy duration calculated as 280 days from LMP. Used universally though only 5% deliver exactly at 40 weeks.

Is 37 weeks full term?

No longer. 37-38 weeks is now "early term." Full term is 39-40 weeks. This changed in 2013 due to better outcomes at 39+ weeks.

Why is 38 weeks not considered full term?

Babies born at 38 weeks have higher risks of breathing problems, jaundice, and NICU admission compared to 39-40 weeks. Important development continues.

Is it better for a baby to be born at 39 or 40 weeks?

Both are ideal. 39-40 weeks have lowest complications. Brain, lungs, and liver complete crucial development between 37-39 weeks.

Why induce at 39 weeks instead of 40?

ARRIVE study showed inducing at 39 weeks reduces C-sections (18.6% vs 22.2%) without increasing complications. Now considered safe option.

Why should you leave two weeks between ultrasounds?

Allows sufficient time to detect measurable growth changes. More frequent scans don't improve accuracy and may cause unnecessary anxiety.

How long does sperm live?

Up to 5 days in female reproductive tract (average 3 days), especially around ovulation. Outside body: minutes to 1 hour depending on conditions.

Due Date Calculator

Calculate your estimated due date based on your last menstrual period (LMP) or conception date using Naegele’s Rule.

Pregnancy

Calculation Method:

Last Menstrual Period (LMP) Conception Date

Last Menstrual Period (LMP): *

Enter the first day of your last menstrual period

Average Cycle Length (days): (optional)

Average is 28 days. Adjust if yours is different.

Our Due Date Calculator helps you estimate when your baby will arrive. Based on Naegele’s Rule, this calculator adds 280 days (40 weeks) to the first day of your last menstrual period (LMP).

You can calculate your due date using either:

Last Menstrual Period (LMP): The most common method, calculating from the first day of your last period.
Conception Date: If you know when conception occurred, typically around ovulation (14 days after LMP).

Why Use This Calculator?

Get an estimated due date quickly and easily
Understand your pregnancy timeline and trimester dates
Track your pregnancy progress with visual indicators
Plan for prenatal appointments and preparations

Important: This calculator provides an estimate only. Only about 5% of babies are born on their exact due date. Your healthcare provider may adjust your due date based on ultrasound measurements.

What This Means

Your estimated due date is calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly.

Remember that only about 5% of babies arrive exactly on their due date. Most babies are born within 2 weeks before or after the estimated due date. Your healthcare provider may give you a more accurate date after an ultrasound examination.

Sources & References

[1]ACOG - Methods for Estimating the Due Date

Due Date Calculator

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Your Estimated Due Date

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First Trimester

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Why Use This Calculator?

What This Means

Sources & References

Frequently Asked Questions

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