Pregnancy Calculator - Due Date, Week & Trimester

The invisible countdown from your last period to delivery day

Pregnancy is dated from a day you were not yet pregnant. The medical clock starts ticking on the first day of your last menstrual period (LMP), roughly two weeks before conception actually occurs. That means when a doctor says "you are 6 weeks pregnant," the embryo is really about 4 weeks old. This calculator applies the Naegele rule - add 280 days (40 weeks) to your LMP - and adjusts for cycle length if yours differs from the standard 28 days. The result: your estimated due date (EDD), current week, trimester and a countdown to delivery.

Trimester overview

Trimester	Weeks	Key developments	Common symptoms
1st trimester	1-13	Implantation, heartbeat (week 6), organs forming, limbs developing	Nausea, fatigue, breast tenderness, frequent urination
2nd trimester	14-27	Gender visible (week 16-20), first movements, anatomy scan, viability threshold (week 24)	Energy returns, appetite increases, visible belly, backache may start
3rd trimester	28-40	Rapid weight gain, lungs maturing, head-down position, full-term at 37 weeks	Braxton Hicks, heartburn, swelling, shortness of breath, nesting instinct

How to use this calculator - step by step

First day of last menstrual period (LMP) - select the date of the first day of your most recent period before becoming pregnant. This is the standard starting point used by doctors worldwide. If you are unsure of the exact date, your healthcare provider can estimate gestational age from an early ultrasound (most accurate between weeks 8-12).
Menstrual cycle length (optional) - if your cycle is not the standard 28 days, enter your typical cycle length (21-35 days). The calculator adjusts the due date: a 32-day cycle pushes the due date 4 days later, a 24-day cycle pulls it 4 days earlier. This correction accounts for later or earlier ovulation.
Read your results - you get the estimated due date (EDD), your current week and day of pregnancy, which trimester you are in, the estimated conception date, days since conception, days remaining until delivery, and stage-specific information about fetal development and common symptoms.

Key pregnancy milestones

Week	Milestone	Details
4	Positive test possible	hCG detectable in urine, implantation complete
6	Heartbeat	Fetal heartbeat detectable on ultrasound (100-160 bpm)
12	End of highest risk period	Miscarriage risk drops to under 2%. First trimester screening (NT scan + blood test)
16-20	First movements (quickening)	First-time mothers often feel movements around week 20, experienced mothers as early as 16
18-22	Anatomy scan	Detailed ultrasound checking all organs, spine, brain, heart. Gender visible
24	Viability threshold	Baby has a chance of survival if born (with intensive NICU care)
28	Third trimester begins	Survival rate above 90% if born now. Glucose tolerance test often done
37	Full-term	Baby is considered full-term. Lungs mature. Safe for delivery
40	Due date	Only about 5% of babies are born on their exact due date

Practical examples

Standard 28-day cycle
LMP: March 1 | Due date: December 6 | Conception: approx. March 15
At 10 weeks: 1st trimester, first prenatal visit and blood work

Short cycle (24 days)
LMP: March 1, cycle 24 days | Due date: December 2 (4 days earlier)
Ovulation occurred around day 10 instead of day 14, so the baby is slightly older than standard dating suggests

Long cycle (35 days)
LMP: March 1, cycle 35 days | Due date: December 13 (7 days later)
Ovulation was delayed to day 21 - without this correction, the due date would be a week too early

Early pregnancy (week 6)
LMP: 6 weeks ago | Current: 6 weeks 0 days | Trimester: 1st
First heartbeat visible on ultrasound. Morning sickness may peak in the coming weeks

Third trimester (week 34)
LMP: 34 weeks ago | Remaining: about 42 days | Trimester: 3rd
Baby weighs approximately 2.1 kg. Head may be engaging. Hospital bag time

FAQ

Why is pregnancy counted from the last period and not from conception?

Most women know when their last period started but not the exact day of conception (ovulation can vary by several days even in regular cycles). The LMP method has been the medical standard for over a century because it provides a consistent, reliable starting point. The trade-off is that "week 1" and "week 2" of pregnancy are before conception even happened. The actual embryonic age is always about 2 weeks less than the gestational age.

How accurate is the due date?

Only about 5% of babies arrive on their exact due date. About 80% are born within 2 weeks of the EDD (between weeks 38 and 42). First babies tend to arrive slightly later than the due date on average. An early ultrasound (weeks 8-12) can refine the due date to within +/- 5 days, which is more accurate than LMP-based calculation if cycle length is irregular or uncertain.

What if I do not know my last period date?

Your healthcare provider can estimate gestational age using an early ultrasound. Between weeks 8-12, measuring the crown-rump length (CRL) of the embryo gives an accurate age estimate within 3-5 days. Later ultrasounds are less precise because individual growth variation increases. If you have a known conception date (e.g., from IVF), subtract 2 weeks to get an equivalent LMP date for this calculator.

Does cycle length really matter for the due date?

Yes, significantly. The Naegele rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is 35 days, ovulation likely occurs around day 21 - a full week later. Without the cycle correction, your due date would be 7 days too early. This can lead to unnecessary induction if doctors believe the pregnancy is overdue when it is actually right on time. Always mention your typical cycle length to your provider.

What is the Naegele rule?

Named after German obstetrician Franz Naegele (1778-1851), the rule states: take the first day of the LMP, add 1 year, subtract 3 months, and add 7 days. This equals 280 days or 40 weeks. Example: LMP = March 1 -> add 1 year (March 1 next year) -> subtract 3 months (December 1) -> add 7 days = December 8. The calculator does this math automatically and adds the cycle-length correction that Naegele's original formula lacked.

When should I first see a doctor?

Most healthcare providers schedule the first prenatal visit between weeks 8-10. At this appointment, you will typically get a dating ultrasound, blood work (blood type, Rh factor, iron, thyroid, infections), blood pressure baseline, and medical history review. If you have a high-risk factor (previous complications, chronic conditions, age over 35), contact your provider as soon as you get a positive test.

What supplements should I take?

Folic acid (400-800 mcg/day) is the most important supplement - ideally started before conception and continued through the first trimester. It reduces neural tube defects by up to 70%. Other common recommendations: iron (27 mg/day, especially in 2nd-3rd trimester), vitamin D (600-1000 IU/day), DHA/omega-3 (200-300 mg/day for brain development), and calcium (1000 mg/day). Always consult your provider before adding supplements.

What happens if I go past my due date?

Going past the due date is common - especially for first pregnancies. Most providers will start monitoring more closely after week 40 (non-stress tests, amniotic fluid checks). Between weeks 41-42, induction is usually discussed. After 42 weeks, the risk of stillbirth, meconium aspiration and placental insufficiency increases, so most guidelines recommend delivery by 42+0. The ARRIVE trial showed that elective induction at 39 weeks for low-risk pregnancies does not increase cesarean rates and may slightly reduce complications.

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