📐 Naegele's Rule Formula:

EDD = First Day of LMP + 280 days (40 weeks)

💡 Example Calculation

LMP: January 15, 2025
Add 1 year → January 15, 2026
Subtract 3 months → October 15, 2025
Add 7 days → Estimated Due Date: October 22, 2025

The first trimester begins on the first day of your LMP. By week 4, the fertilized egg has implanted in the uterus. By week 8, your baby (now called an embryo) has a heartbeat. By week 12, the embryo becomes a fetus with all major organs formed.

Common symptoms: Morning sickness, fatigue, breast tenderness, frequent urination, food aversions.

Key appointments: Confirmation of pregnancy, first prenatal visit, nuchal translucency scan (weeks 11–13), NIPT blood test.

Often called the "golden trimester," most women feel their best during this period. Morning sickness typically eases, energy returns, and you'll start to show. Your baby grows rapidly — by week 20 you can often find out the sex during the anatomy scan. Around week 18–22, you'll feel your baby's first movements (quickening).

Common symptoms: Growing belly, back pain, round ligament pain, skin changes, increased appetite.

Key appointments: Anatomy ultrasound at week 20, quad screen blood test, gestational diabetes screening (weeks 24–28).

The final stretch. Your baby is gaining weight rapidly and preparing for life outside the womb. By week 37, your baby is considered full term. Many women experience Braxton Hicks contractions, increased pressure in the pelvis, and disrupted sleep. Your prenatal visits become more frequent (weekly after week 36).

Common symptoms: Braxton Hicks contractions, swelling, heartburn, shortness of breath, difficulty sleeping.

Key appointments: Group B Strep test (weeks 35–37), biophysical profile if needed, weekly NST (non-stress test) after 40 weeks.

Week	Milestone	Baby's Size
Week 1–4	Fertilization, implantation, pregnancy test turns positive	Poppy seed
Week 6	Heartbeat detectable by ultrasound; neural tube forming	Lentil
Week 8	All major organs have begun forming; baby moves (you can't feel it yet)	Raspberry
Week 12	End of first trimester; nuchal scan; risk of miscarriage drops significantly	Lime
Week 16	Baby can make facial expressions; some parents feel first flutters	Avocado
Week 20	Anatomy scan; can often find out baby's sex; halfway point!	Banana
Week 24	Viability milestone — baby has a chance of survival if born early	Corn
Week 28	Start of third trimester; baby opens eyes; lungs maturing	Eggplant
Week 32	Baby practicing breathing; gaining ~½ lb per week; head may engage	Squash
Week 36	Baby is nearly full-sized; weekly OB visits begin; Group B Strep test	Romaine lettuce head
Week 37	Early term — baby is considered term; most systems ready	Winter melon
Week 39–40	Full term — ideal delivery window; your estimated due date falls here	Small pumpkin
Week 41+	Late/post-term; OB may discuss induction; increased monitoring	Large pumpkin

🔬 Gold Standard: First-Trimester Ultrasound

A dating ultrasound performed between 8 and 13 weeks of pregnancy is the most accurate way to confirm your due date. The sonographer measures the crown-rump length (CRL) of the embryo, which can estimate gestational age to within ±5–7 days. If the ultrasound date differs significantly from your LMP-based EDD, your doctor will typically adjust your due date to match the ultrasound.

EDD = LMP + 280 days + (Your cycle length − 28 days)

How is a pregnancy due date calculated?

The most widely used method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. Your doctor may refine this estimate based on a first-trimester ultrasound, which is the most accurate way to date a pregnancy.

I'm 6 weeks pregnant — when is my due date?

If you're currently 6 weeks pregnant, you're about 6 weeks from the start of your last menstrual period. You have approximately 34 more weeks until your due date. Use the calculator above: enter your LMP date, and it will give you your exact EDD instantly.

What if I don't know my last period date?

No problem. Use the Conception Date option in our calculator if you know when you conceived. Alternatively, visit your doctor for an early ultrasound — this can estimate your gestational age and due date based on fetal measurements, regardless of whether you know your LMP.

Can my due date change after it's set?

Yes, though it's most likely to be adjusted in the first trimester. If your early ultrasound shows a significant difference from your LMP-based EDD (usually more than 5–7 days before 14 weeks), your provider will update your due date. After 20 weeks, due dates are rarely changed by ultrasound alone.

What does "full term" mean in pregnancy?

Pregnancy term classifications: Early term is 37–38 weeks, full term is 39–40 weeks (the ideal delivery window), late term is 41 weeks, and post-term is 42 weeks or beyond. Babies born before 37 weeks are preterm. Your EDD falls at the 40-week full-term mark.

Is a due date calculator accurate for irregular periods?

Standard LMP-based calculators are less accurate if your cycles are irregular, shorter, or longer than 28 days. In those cases, a first-trimester ultrasound is especially important to confirm gestational age. Some calculators (like ours) allow cycle-length adjustments to improve accuracy.

How many weeks pregnant am I right now?

Enter your LMP date into the calculator above — it will instantly tell you how many weeks and days pregnant you are, which trimester you're in, and your estimated due date. Gestational age is counted from your LMP, not your conception date.

Do babies always come on their due date?

Rarely — only about 4–5% of babies are born on their exact due date. The vast majority arrive within 2 weeks before or after the EDD. Labor between 37 and 42 weeks is considered normal. If you haven't delivered by 41–42 weeks, your healthcare provider may discuss induction options with you.