Question 1

When did I conceive?

Accepted Answer

For a standard 28-day cycle, conception occurs on day 14 — about 2 weeks after your last menstrual period (LMP) started. If you know your due date instead, subtract 266 days (38 weeks) to estimate conception. Our conception calculator runs both directions: enter LMP, due date, or ultrasound dating to get the conception date instantly. For irregular cycles, adjust the cycle length input — ovulation is always ~14 days before the next expected period.

Question 2

How accurate is a conception calculator?

Accepted Answer

Conception calculators are accurate within 5-7 days for women with regular 28-30 day cycles. The biggest sources of error: (1) irregular cycle length — if you ovulated earlier or later than day 14, the estimate shifts; (2) confusing the date of sex with the date of fertilization — sperm can survive 3-5 days, so conception may happen days after intercourse; (3) using LMP when ovulation timing differs significantly. For best accuracy, use ultrasound dating in the first trimester — it's accurate within 3-5 days.

Question 3

Can I calculate conception date from my due date?

Accepted Answer

Yes — and this is the most common use case. Subtract 266 days (38 weeks) from your due date to estimate the conception date, OR subtract 280 days (40 weeks) to find your LMP and then add 14 days. Our conception date calculator from due date does this automatically when you select 'From due date' mode. Note: this assumes a standard 28-day cycle and Naegele's rule. For non-standard cycles, adjust the cycle length input.

Question 4

How long after ovulation does conception happen?

Accepted Answer

Fertilization (the moment sperm meets egg) happens within 12-24 hours of ovulation — the egg's viability window is short. However, sperm can survive 3-5 days in fertile cervical mucus, so sex that happens UP TO 5 DAYS BEFORE ovulation can still result in conception when the egg is released. This is why the fertile window is 5-6 days long even though ovulation itself is a single ~24-hour event. Conception 'date' in calculators typically refers to the ovulation date, not the day of intercourse.

Question 5

Is conception date the same as fertilization date?

Accepted Answer

Clinically, yes — both refer to the moment the sperm penetrates the egg, which happens within 24 hours of ovulation. But colloquially, people often confuse 'conception date' with 'the day we had sex'. These can be different by 1-5 days because sperm survives in the fertile cervical mucus before reaching the egg. Our conception calculator outputs the most likely fertilization day based on ovulation timing, NOT the day of intercourse.

Question 6

Can ultrasound determine conception date?

Accepted Answer

Yes — and ultrasound is more accurate than LMP-based dating, especially in the first trimester. A dating ultrasound performed between 8 and 13 weeks measures the embryo's crown-rump length (CRL), which correlates tightly with gestational age (±3-5 days accuracy). If the ultrasound dating differs from LMP dating by more than 7 days in the first trimester, doctors typically use the ultrasound date. Our calculator's 'From ultrasound' mode takes the scan date and reported gestational age and reverse-calculates the conception date.

Question 7

How is conception date estimated?

Accepted Answer

There are three standard methods. (1) From LMP: conception = first day of last period + (cycle length − 14). For a typical 28-day cycle, that's day 14. (2) From due date: conception = due date − 266 days (38 weeks). (3) From ultrasound: subtract the gestational age (in days) from the ultrasound date to get LMP, then add 14 days. Our conception estimator runs all three on demand — pick whichever input you have most confidently and the result is the same calc the OB/midwife uses.

Question 8

What is the fertile window?

Accepted Answer

The fertile window is the 5-6 day period each cycle when pregnancy is possible. It includes the 5 days BEFORE ovulation (when sperm deposited can survive and wait for the egg) and the day of ovulation itself. Peak fertility is the 2-3 days immediately before ovulation — that's when intercourse is most likely to result in conception. Our conception window calculator estimates this range based on your cycle length and ovulation timing. Use our [ovulation calculator](/ovulation-calculator) for cycle-forward planning.

Question 9

How many days after conception does pregnancy begin?

Accepted Answer

Clinically, pregnancy is dated from the first day of the last menstrual period (LMP), so 'pregnancy' technically begins about 2 weeks BEFORE conception in obstetric counting. Biologically, however, the embryo starts developing at fertilization (conception). Implantation — when the embryo attaches to the uterine lining — happens 6-12 days after conception. Most pregnancy tests detect hCG hormone starting 10-14 days after conception, around the time of the missed period.

Question 10

Can conception calculators be wrong?

Accepted Answer

Yes. Common error sources: (1) Irregular cycles — if your cycle varies by more than 7 days, ovulation timing varies and LMP-based estimates drift. (2) Late or early ovulation in a normal cycle — even a 'regular' 28-day cycle can have ovulation on day 12 or day 16, shifting conception. (3) Lactational anovulation — postpartum mothers may not ovulate at day 14 of returning cycles. (4) PCOS, thyroid issues, or stress that disrupt ovulation timing. Ultrasound dating in the first trimester is the most accurate method available and is the gold standard for resolving discrepancies.

Question 11

What's the difference between conception and implantation?

Accepted Answer

Conception (fertilization) happens within 24 hours of ovulation — the sperm penetrates the egg and forms a zygote. Implantation happens 6-12 days LATER — the resulting blastocyst travels down the fallopian tube and embeds in the uterine wall. hCG hormone production starts at implantation, which is why home pregnancy tests work earliest 10-14 days post-conception. 'Implantation bleeding' (light spotting) sometimes occurs at implantation and is often mistaken for a light period.

Question 12

How is IVF conception date calculated differently?

Accepted Answer

IVF pregnancies use a different dating convention because the conception date is precisely known (the day of egg retrieval and fertilization) and the embryo is transferred days later (day 3 or day 5 typically). Standard IVF dating: due date = transfer date + 263 days (for day-5 blastocyst transfer) or + 261 days (for day-3 embryo transfer). LMP is calculated backwards: due date − 280 days. Our standard conception calculator doesn't handle IVF — use our [pregnancy due date calculator](/pregnancy-due-date-calculator) with IVF mode for accurate IVF dating.

Question 13

Why does conception happen 2 weeks AFTER my last period?

Accepted Answer

It doesn't happen at exactly 2 weeks for everyone — that's just the average for a 28-day cycle. Here's the reason: the menstrual cycle has two phases. The follicular phase (from LMP to ovulation) varies in length between women — anywhere from 10 to 20+ days. The luteal phase (from ovulation to next period) is almost always 14 days (±1) regardless of cycle length. So if your cycle is 32 days, you likely ovulate around day 18, not day 14. Our conception calculator adjusts for this when you input your actual cycle length.

Question 14

How does Naegele's rule work?

Accepted Answer

Naegele's rule is the standard obstetric formula for due date calculation, used worldwide since 1812. It calculates: due date = LMP + 1 year − 3 months + 7 days. For a January 1 LMP, that's October 8 of the same year. Equivalent to adding 280 days. The rule assumes a 28-day cycle with ovulation at day 14. Most countries (US ACOG, UK NHS, Canada SOGC, Australia RANZCOG) use Naegele's rule as the default, then refine with ultrasound dating in the first trimester if available.

Question 15

What's the difference between gestational age and fetal age?

Accepted Answer

Gestational age is the standard obstetric measure: weeks since the first day of the last menstrual period (LMP). At 'week 8 of pregnancy', the embryo is gestational age 8 weeks (counted from LMP). Fetal age (embryonic age) is counted from conception, which is ~2 weeks AFTER LMP — so 'week 8 of pregnancy' = 6 weeks of actual fetal development. Doctors, ultrasounds, and pregnancy apps almost always use GESTATIONAL age (LMP-based) because conception date is rarely known with certainty.

Question 16

Can I use this conception calculator without knowing my LMP?

Accepted Answer

Yes — that's why we offer three modes. If you don't know your LMP (common after stopping birth control, postpartum, or with very irregular cycles), use the 'From due date' mode if you have a due date from your provider, OR the 'From ultrasound' mode if you've had a dating scan. Both modes bypass the need for an accurate LMP. The 'From ultrasound' mode is the most accurate option in early pregnancy.

Question 17

How does the conception calculator handle twins?

Accepted Answer

Same as singleton pregnancies for conception date calculation. Conception happens in a single cycle whether 1 egg is fertilized or 2 (fraternal twins) — both events occur at the same ovulation. Identical twins arise from a single fertilized egg splitting days later, also tied to the same conception event. For dating purposes, twin pregnancies use the same 280-day Naegele's rule with the same conception date. What differs is the recommended delivery timing (37-38 weeks for twins vs 40 weeks for singletons), but that's a delivery decision, not a dating one.

Question 18

Is the conception calculator different in the UK, Canada, and Australia?

Accepted Answer

The math is identical worldwide — Naegele's rule (280-day pregnancy from LMP, conception at LMP + 14 for a 28-day cycle) is the global obstetric standard adopted by ACOG (US), RCOG/NHS (UK), SOGC (Canada), and RANZCOG (Australia). What differs slightly is which trimester boundary system clinicians use: most authorities define trimester 1 = weeks 0-12, trimester 2 = 13-27, trimester 3 = 28-40, but some sources use 0-13 / 14-26 / 27-40. Our calculator uses the more common 0-12 / 13-26 / 27-40 boundaries.

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