2026-08-07 10 min

Pregnancy Conception Calculator

Estimate your conception date, fertile window, implantation window, due date, gestational age, fetal age, and trimester from the first day of your last period with Naegele rule and ovulation-offset math.

Calculator

Last Period (LMP)

Cycle Length

Cycle Details

Luteal Phase

Confirm LMP

Results update automatically

Results update live as you type

Status:Conception May 15, 2026Second Trimester (Weeks 14–27)Due Feb 5, 2027

Pregnancy Conception Summary

LMP2026-05-01

Conception DateMay 15, 2026

Due DateFeb 5, 2027

Gestational Age25 weeks 3 days

Fertile WindowMay 10, 2026 – May 16, 2026

Pregnancy Progress

Gestational Progress178 / 280 days

Time Remaining102 days left

Cycle Position of OvulationDay 14 / 28

Key Dates

Fertile Window:May 10, 2026 – May 16, 2026

Implantation Window:May 21, 2026 – May 25, 2026

Trimester:Second Trimester (Weeks 14–27)

Milestone Dates

Milestone	Date	Day
End of First Trimester (13w 6d)	Aug 6, 2026	d97
Viability Milestone (24w 0d)	Oct 16, 2026	d168
Full Term (37w 0d)	Jan 15, 2027	d259
Due Date	Feb 5, 2027	d280

Results

Optimal

Conception Date

May 15, 2026

LMP + 14 days (cycle 28 − luteal 14)

Due Date

Feb 5, 2027

LMP + 280 days (Naegele rule)

Gestational Age

25 weeks 3 days

Days since LMP, measured today

Trimester

Second Trimester (Weeks 14–27)

First 1–13, second 14–27, third 28–40 weeks

Fertile Window

May 10, 2026 – May 16, 2026

Ovulation − 5 days to ovulation + 1 day

Fetal Age

23 weeks 3 days

Time since conception (gestational − ~14 days)

Implantation Window

May 21, 2026 – May 25, 2026

Conception + 6 to + 10 days

Time Remaining

102 days (14 weeks)

Days from today until the due date

LMP-based conception estimates are typically accurate within several days for regular cycles; ovulation timing varies, so first-trimester ultrasound dating (accurate to about 5–7 days) is more reliable, especially with irregular cycles.

Visualizations

Pregnancy Timeline Offsets (days from LMP)

14dOvulation

22dImplantation

97dEnd T1

280dDue Date

Gestational Progress

Elapsed63.6%

Cycle Day of Ovulation

Ovulation14d

Formula

The conception estimate offsets the LMP by the follicular phase (cycle length minus the ~14-day luteal phase), because ovulation precedes the next period by a fairly constant luteal span. The due date adds the standard 280-day (40-week) gestation to the LMP. The fertile window covers the five days sperm can survive before ovulation plus ovulation day, and implantation follows conception by 6–10 days.

Conception = LMP + (Cycle − Luteal); Due = LMP + 280 days

Alternative Form

Naegele shortcut: Due = LMP − 3 months + 7 days + 1 year (≈ LMP + 280 days)

Variables

Symbol	Name	Unit	Description
LMP	Last menstrual period	date	First day of the last period; day zero of gestational age
C	Cycle length	days	Average menstrual cycle, 21–45 days (default 28)
L	Luteal phase	days	Ovulation-to-period span, usually ~14 days
O	Ovulation offset	days	C − L; cycle day of ovulation and conception
D	Due date	date	LMP + 280 days (Naegele rule)
F	Fertile window	dates	Conception − 5 days through conception + 1 day
G	Gestational age	days	Today − LMP; fetal age is G − 14

Explanation

Gestational age counts from the LMP because ovulation is invisible but the period is observable; the cycle-minus-luteal offset reconstructs the hidden ovulation day. Adding 280 days converts that clock into a due date, while the fertile and implantation windows bracket the biology around conception.

Derivation

Conception = LMP + (Cycle − Luteal). Due = LMP + 280. Fertile Start = Conception − 5; Fertile End = Conception + 1. Implantation = Conception + 6 … + 10. Gestational Days = Today − LMP. Fetal Days = Gestational − 14. Trimester: ≤13w first, 14–27w second, ≥28w third. Progress = Gestational ÷ 280 × 100.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Start with the first day of the last menstrual period and validate it as a real calendar date.

LMP must match YYYY-MM-DD in UTC

LMP Input:2026-05-01

Parsed:2026-05-01

Intermediate: 2026-05-01 accepted as day zero of the pregnancy clock

Result: LMP = Friday, May 1, 2026

Step 2

Estimate ovulation and conception from the cycle length minus the luteal phase.

Conception = LMP + (Cycle − Luteal)

Cycle Length:28 days

Luteal Phase:14 days

Offset:14 days

Intermediate: 2026-05-01 + 14 days

Result: Conception = Friday, May 15, 2026

Step 3

Apply Naegele rule: add 280 days (40 weeks) to the LMP for the estimated due date.

Due Date = LMP + 280 days

LMP:2026-05-01

Added Days:280

Intermediate: 2026-05-01 + 280 days

Result: Due date = Friday, February 5, 2027

Step 4

Measure gestational age today and derive fetal age by subtracting ~14 pre-ovulation days.

Gestational = Today − LMP; Fetal = Gestational − 14

Gestational Days:178

Fetal Days:164

Intermediate: 178 days since LMP; 164 days since conception

Result: Gestational 25 weeks 3 days; fetal 23 weeks 3 days

Step 5

Bracket conception with the fertile window (ovulation − 5 to + 1) and the implantation window (conception + 6 to + 10).

Fertile = C − 5 … C + 1; Implantation = C + 6 … C + 10

Conception:2026-05-15

Fertile Window:2026-05-10 to 2026-05-16

Intermediate: Fertile May 10, 2026 – May 16, 2026; implantation May 21, 2026 – May 25, 2026

Result: A six-day conception window plus a five-day implantation window

Step 6

Assign the trimester from gestational weeks and project the next three milestones.

T1 ≤ 13w; T2 14–27w; T3 ≥ 28w

Gestational Weeks:25

Trimester:Second Trimester (Weeks 14–27)

Intermediate: End of First Trimester (13w 6d): 2026-08-06; Viability Milestone (24w 0d): 2026-10-16; Full Term (37w 0d): 2027-01-15

Result: Second Trimester (Weeks 14–27) with 102 days (14 weeks) remaining

Final Answer

May 15, 2026

Worked Examples

See how the calculator works with different scenarios.

Scenario

LMP on 2026-05-01 with a textbook 28-day cycle and 14-day luteal phase.

Inputs

LMP:2026-05-01

Cycle Length:28 days

Luteal Phase:14 days

Calculation

Conception = May 1 + (28 − 14) = May 15, 2026 Due = May 1 + 280 days = Feb 5, 2027 Fertile = May 10 – May 16, 2026

Result

Conception May 15, 2026; due Feb 5, 2027

Interpretation

The classic textbook case: ovulation on cycle day 14, a six-day fertile window straddling mid-May, and a February due date 280 days after the LMP.

Understanding Your Result

Your Result: Conception Date: May 15, 2026

Typical range: Conception usually falls 11–21 days after LMP (cycles 25–35 days). Gestation runs 0–280 days to the due date; healthy births span roughly 259–294 days (37–42 weeks).

What This Means

Every figure here is anchored to your LMP: the conception estimate, the fertile and implantation windows around it, the 280-day due date, and today's gestational and fetal ages with the current trimester.

Why It Matters

Knowing the likely conception window clarifies paternity timing questions, guides when to test, and sets the schedule for prenatal screening windows that depend on gestational age.

Professional Interpretation

Providers treat LMP dates as provisional and confirm with first-trimester ultrasound; a discrepancy over ~5–7 days typically re-dates the pregnancy to the scan.

Recommendations

•Confirm LMP dates with a first-trimester ultrasound
•Track ovulation directly when cycles are irregular
•Test for pregnancy around the missed period, not earlier
•Book prenatal care once a home test is positive
•Bring cycle history to the dating appointment
•Re-run the estimate if a scan revises the due date

Educational Content

Introduction

The Pregnancy Conception Calculator reconstructs the hidden day of ovulation from the visible LMP using the cycle-minus-luteal offset, then projects the due date, fertile and implantation windows, and today's gestational position on the 40-week clock.

Concept Explanation

Because the luteal phase is nearly constant (~14 days) while the follicular phase varies, ovulation day equals cycle length minus luteal length. Adding that offset to the LMP estimates conception; adding 280 days gives the Naegele due date; subtracting the LMP from today gives gestational age.

Applications

▸Estimating the conception date from LMP and cycle length

▸Identifying the six-day fertile window for timing intercourse

▸Predicting the implantation window and earliest test dates

▸Computing the Naegele-rule due date (LMP + 280 days)

▸Tracking gestational age, fetal age, and trimester today

▸Planning prenatal milestones from viability to full term

Benefits

•Converts a remembered period date into a full pregnancy timeline
•Adjusts automatically for long, short, or irregular cycles
•Separates gestational age from fetal age clearly
•Flags fertile and implantation windows for planning
•Milestones keep screening and viability dates in view

Limitations

•Assumes ovulation timing from averages, not direct observation
•Less reliable with irregular cycles or uncertain LMP recall
•Cannot fix the exact day sperm met egg within the window
•Ultrasound dating supersedes LMP math after the first trimester

Best Practices

•Enter the true first bleeding day, not spotting, as LMP
•Use your average cycle over 3–6 months, not one outlier
•Keep the luteal default unless tracking proves otherwise
•Confirm every LMP estimate with an early ultrasound
•Re-check dates after any bleeding or hormonal disruption

Common Misconceptions

Myth: Conception happens on one knowable day.

Truth: Sperm survival spreads possible conception across the fertile window; the calculator reports the most likely ovulation day, not a certifiable hour.

Myth: Everyone ovulates on day 14.

Truth: Day 14 only fits 28-day cycles. A 35-day cycle ovulates near day 21 and a 21-day cycle near day 7 — the offset math corrects for this.

Myth: Gestational age equals fetal age.

Truth: Gestational age includes ~14 pre-ovulation days, so the fetus is about two weeks younger than the gestational week count suggests.

Myth: The due date is the delivery date.

Truth: Only ~5% of babies arrive on the due date; 37–42 weeks is the normal range, which is why the calculator shows windows and milestones, not a single destiny.

Practical Tips

•Record the first full-flow day as LMP, not spotting
•Average cycle length over several months for stability
•Start ovulation tracking early with short cycles
•Time intercourse to the 1–2 days before ovulation
•Wait until the missed period for the most reliable test
•Confirm LMP dates with a first-trimester scan
•Re-run the calculator after any dating ultrasound

Common Mistakes

Assuming day-14 ovulation for every cycle

Enter your real average cycle length so the cycle-minus-luteal offset places ovulation correctly.

Using spotting instead of full flow as LMP

Day one is the first day of full menstrual flow; spotting beforehand shifts every downstream date.

Testing days before the missed period

hCG is reliably detectable around 12–14 days after conception; earlier tests often false-negative.

Confusing gestational and fetal age

Subtract ~14 days from gestational age to get fetal age when comparing to conception-based references.

Treating the due date as exact

Plan around the 37–42 week range and milestone windows rather than a single calendar day.

Assumptions

This calculator makes the following assumptions to provide estimates:

Regular Cycles: Cycle length input reflects the recent average; ovulation recurs predictably.

Stable Luteal Phase: The luteal span is near-constant (~14 days) across cycles.

Accurate LMP Recall: The entered LMP is the true first day of full menstrual flow.

UTC Day Math: All dates use UTC midnight arithmetic to avoid timezone shifts.

No Intervening Disruption: No hormonal contraception, miscarriage, or anovulatory cycle distorts the current clock.

Limitations

Be aware of these limitations when using this calculator:

Estimate, Not Observation: Ovulation is inferred from averages; direct tracking or scans are definitive.

Irregular Cycle Noise: Variable ovulation widens the true window beyond the reported dates.

LMP Recall Error: A misremembered LMP shifts conception, due date, and age together.

Ultrasound Supersedes: First-trimester crown-rump-length dating replaces LMP math when they disagree.

Frequently Asked Questions

Answers to common questions about this calculator.

When did I conceive?

Based on your LMP and cycle length, this calculator estimates conception at LMP plus (cycle length minus luteal phase) days — about day 14 in a 28-day cycle. Conception typically occurs within 12–24 hours of ovulation.

How accurate is this conception date estimate?

LMP-based estimates are usually accurate within several days for regular cycles, but ovulation timing varies. First-trimester ultrasound dating (accurate to about 5–7 days) is more reliable, especially with irregular cycles.

What is the difference between the conception date and the due date?

The conception date is when the egg was fertilized. The due date is about 38 weeks (266 days) after conception, or 40 weeks (280 days) after the first day of the LMP.

What is the fertile window?

The fertile window spans about six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in the reproductive tract, while the egg is viable only 12–24 hours after ovulation.

How long after intercourse does conception occur?

Conception can occur within minutes to hours if ovulation already happened, or up to five days later if sperm arrive first and wait in the reproductive tract for the egg to be released.

When does implantation occur after conception?

Implantation typically occurs 6–10 days after ovulation and conception, when the blastocyst embeds in the uterine lining and pregnancy officially begins with rising hCG.

How do I calculate the conception date from my LMP?

Add (cycle length − 14) days to the first day of your LMP. For a 28-day cycle that is LMP + 14 days; for a 35-day cycle it is LMP + 21 days. This calculator does it automatically, including a custom luteal phase.

How is the due date calculated (Naegele rule)?

The due date is LMP + 280 days (40 weeks). The classic Naegele shortcut — subtract 3 months, add 7 days and 1 year — approximates the same 280-day span.

What is the difference between gestational age and fetal age?

Gestational age is measured from the LMP (about 40 weeks total) and is the standard providers use. Fetal age is measured from conception (about 38 weeks), roughly two weeks less than gestational age.

Can I conceive during my period?

It is unlikely but possible: sperm can survive up to five days, and with a short cycle ovulation can follow soon after bleeding ends, so intercourse late in a period can occasionally lead to pregnancy.

What is the best time to conceive?

The one to two days before ovulation give the highest probability, since sperm are already present when the egg is released. In a 28-day cycle the fertile window usually falls around cycle days 9–15.

How long does sperm survive in the female body?

Up to five days under optimal fertile cervical mucus conditions, though most sperm lose fertilizing ability after about three days.

How long does an egg survive after ovulation?

Only about 12–24 hours. If fertilization does not occur in that window the egg degenerates, which is why timing intercourse to the fertile window matters.

Can I conceive with irregular periods?

Yes, but predicting ovulation from LMP alone is less reliable. Track ovulation with predictor kits, basal body temperature, or cervical mucus, and confirm dating with an early ultrasound.

How does cycle length change the conception estimate?

Longer cycles push ovulation later (a 35-day cycle ovulates around day 21), while shorter cycles pull it earlier (a 21-day cycle around day 7). This calculator shifts the estimate automatically via the cycle-minus-luteal offset.

What is the luteal phase and why does it matter?

The luteal phase is the span from ovulation to the next period, usually about 14 days and far more consistent than the follicular phase. Entering your own value fine-tunes the ovulation offset.

How does IVF affect conception timing?

With IVF, fertilization timing is known precisely: eggs are retrieved about 36 hours after the trigger shot and fertilized within hours, so the conception date and embryonic age are exact rather than estimated.

What factors affect conception date accuracy?

Cycle irregularity, variable ovulation timing, sperm survival up to five days, occasional double ovulation, and implantation variation of a few days all widen the true conception window.

When should I take a pregnancy test after conception?

Home tests are most reliable after a missed period, about 12–14 days after conception. Sensitive tests may detect hCG 6–8 days after ovulation but false negatives are common that early.

What is implantation bleeding?

Light pinkish or brown spotting lasting a day or two, occurring 6–10 days after ovulation when the blastocyst implants. It is lighter and shorter than a period and not everyone experiences it.

Can stress or lifestyle affect conception?

Severe chronic stress, smoking, heavy alcohol use, and very low or high BMI can disrupt ovulation and fertility, while ordinary day-to-day stress does not typically prevent conception.

How does age affect conception?

Female fertility peaks in the early to mid-twenties and declines gradually after 30, more steeply after 35. Per-cycle conception probability falls from roughly 20–30% at 25 to about 5–10% by 40.

Can twins change the estimated dates?

Fraternal twins come from two eggs in one cycle and identical twins from one split embryo; conception timing matches singleton pregnancies, though twins are often born earlier, which can shift the actual delivery away from the estimate.

When should I confirm these dates with my provider?

Confirm as soon as pregnancy is suspected. Providers typically verify LMP-based dates with a first-trimester ultrasound (crown-rump length), the gold standard for dating, and adjust the due date if the discrepancy exceeds about 5–7 days.

Related Calculators

Explore more calculators related to this topic.

Related Guides

Deepen your understanding with these educational resources.

Trying to Conceive: Timing Guide

Fertile windows and ovulation tracking basics.

Understanding Your Due Date

Naegele rule, ultrasounds, and date adjustments.

First Trimester Milestones

Screening windows and early appointments.

Pregnancy Testing Explained

hCG timing and when a test turns positive.

Glossary

Technical terms used in mortgage calculations explained in plain language.

LMP: First day of the last menstrual period; day zero of gestational age.
Ovulation: Release of a mature egg, typically luteal-phase days before the next period.
Conception (fertilization): Sperm-egg fusion in the fallopian tube within ~24 hours of ovulation.
Naegele rule: Due date = LMP + 280 days (40 weeks), the standard dating convention.
Gestational age: Pregnancy age counted from the LMP, about 40 weeks to the due date.
Fetal age: Age since conception, about two weeks less than gestational age.
Trimester: Three pregnancy phases: weeks 1–13, 14–27, and 28–40 gestational.
Fertile window: The ~6 days ending on ovulation day when intercourse can conceive.
Implantation: Blastocyst embedding in the uterine lining, ~6–10 days after conception.
Luteal phase: Post-ovulation cycle span, usually ~14 days and highly consistent.

Authoritative References

Trusted sources and standards referenced by this calculator.

ACOG — Methods for Estimating the Due Date (Practice Bulletin No. 97) CDC — Pregnancy and Reproductive Health NIH MedlinePlus — Pregnancy WHO — Recommendations on antenatal care for a positive pregnancy experience (2020) Mayo Clinic — How Conception Works Jukic et al. 2013 — Length of human pregnancy and contributors to its natural variation Wilcox et al. 1995 — Timing of intercourse in relation to ovulation (NEJM) Robinson & Fleming 1975 — Sonar crown-rump length measurements

Applicable Standards

ACOG Practice Bulletin No. 97: Methods for Estimating the Due DateWHO recommendations on antenatal care for a positive pregnancy experience

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Page Information

Last Updated:

2026-08-07

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.