How accurate is the due date?

Doctors call it an *Estimated* Due Date (EDD) for a reason. Only about 4% of babies are born on their exact due date. Most (80%) are born within the two weeks before or after. It is a target window, not a deadline.

What is Naegele's Rule?

It is the standard formula used by doctors. Take the first day of your Last Menstrual Period (LMP), add 7 days, and then subtract 3 months (or add 9 months). For example, if your LMP was April 1, your due date is January 8.

Can an ultrasound change my date?

Yes. Early ultrasounds (before 13 weeks) are more accurate than the LMP method because they measure the Crown-Rump Length (CRL) of the fetus. If the ultrasound date differs by more than 5-7 days from the LMP date, your doctor will officially move your due date.

Why does pregnancy start before I conceived?

Because it's impossible to know the exact moment of fertilization. The Last Menstrual Period (LMP) is a reliable visible date, so doctors count from there. This means during Week 1 and Week 2 of your 'pregnancy', you aren't actually pregnant yet!

Counting Down: How Your Due Date is Calculated

The 40-week countdown begins before you are even pregnant. Confused? It's all about Naegele's Rule and the mystery of ovulation.

Calculate Your Official EDD

Enter the first day of your last period to see your baby's expected arrival.

Open Pregnancy Calculator

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The "First Two Weeks" Paradox

If you take a pregnancy test and it comes back positive, your doctor might tell you that you are "4 weeks pregnant."But you likely only conceived 2 weeks ago.

Pregnancy is traditionally dated from the first day of your Last Menstrual Period (LMP). Why? Because most women menstruate noticeably but ovulate silently. The LMP is a reliable calendar event. So, Weeks 1 and 2 comprise your period and the ovulation phase. You are not biologically pregnant during this time.

The Math: Naegele's Rule

This 200-year-old formula by German obstetrician Franz Karl Naegele is still the gold standard used worldwide.

LMP + 7 Days - 3 Months = Due Date

Example:
LMP: November 1st
Add 7 Days: November 8th
Subtract 3 Months: August 8th (of the next year)

This formula assumes a standard 28-day cycle with ovulation occurring exactly on Day 14.

Adjustments for Cycle Length

Not every woman has a 28-day cycle.
• Short Cycle (21 days): You likely ovulate earlier (Day 7). Your due date might be 7 days earlier than the rule predicts.
• Long Cycle (35 days): You likely ovulate later (Day 21). Your due date might be 7 days later.

Timeline of the 40 Weeks

Pregnancy is a marathon. Here is how the trimesters break down:

First Trimester (Weeks 1 - 12)

Theme: Rapid Development (and Nausea)
The baby goes from a single cell to a fetus with fingers, toes, and a beating heart. The risk of miscarriage is highest here.

Second Trimester (Weeks 13 - 27)

Theme: Growth & Gender
Often called the "Honeymoon Phase." Nausea usually subsides, energy returns. You will feel the first kicks ("quickening") around week 18-20. The anatomy scan happens around week 20.

Third Trimester (Weeks 28 - 40+)

Theme: Weight Gain & Preparation
The baby packs on fat, the lungs mature, and the head turns downward. Discomfort increases for the mother.

New Definitions of "Full Term"

Doctors used to consider anything past 37 weeks "safe." The American College of Obstetricians and Gynecologists (ACOG) changed this in 2013 because those last two weeks matter for brain and lung development.

Category	Weeks	Outcome
Early Term	37w 0d - 38w 6d	Safe, but not optimal.
Full Term	39w 0d - 40w 6d	Golden window for birth.
Late Term	41w 0d - 41w 6d	Placenta efficiency starts dropping.
Post Term	42w 0d +	High risk involving induction.

Conclusion

Your due date is not an expiration date. It is a midpoint. Trust your care provider, listen to your body, and pack your hospital bag by week 36 just in case!