Calculate your exact conception date from your due date, last menstrual period, IVF transfer date, or ultrasound — plus ovulation windows, IVF dating, and week-by-week fetal development.
One of the most common questions in early pregnancy — asked in doctor's offices, whispered in family groups, and typed into search engines millions of times every month — is simply: when did I conceive?
It is a question that carries significant emotional weight. For couples who planned their pregnancy, it pins down a moment of hopeful possibility. For those surprised by a positive test, it helps establish a timeline. For women who have been through IVF, it can be calculated with remarkable precision. And for anyone preparing for their first antenatal scan, it forms the foundation of everything that follows — gestational age, due date, trimester milestones, and the entire schedule of scans and tests that make up antenatal care.
This guide explains exactly how conception date is calculated — from every starting point — and what the result means for your pregnancy.
Conception is the moment a sperm successfully fertilises an egg, creating a zygote — the single cell from which every cell in a human body will ultimately develop. This event, called fertilisation, typically occurs in the fallopian tube within 12–24 hours of ovulation.
The conception date and the due date (EDD) are fundamentally different things — but they are mathematically linked. The EDD is calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP). But conception does not occur on the first day of the period — it occurs around 14 days later, at ovulation, in a standard 28-day cycle.
So the relationship is:
This is why obstetricians measure gestational age from LMP (not conception) — most women know when their last period started, but very few know exactly when they ovulated or conceived.
There is no single "best" method — the right approach depends on what information you have available. The RxMedCalc Conception Date Calculator supports all four methods.
Fastest method. Reverse-calculate using EDD − 266 days. Works whether due date came from LMP or ultrasound.
Most common. Add 14 days to LMP (adjusted for cycle length). Best for regular cycles.
Most precise. Fertilisation date known. Calculated from transfer date minus embryo age.
Most accurate in early pregnancy. Crown-rump length at 7–13 weeks gives conception date within ±3 days.
If your doctor has already told you your estimated due date — whether based on LMP or confirmed by ultrasound — you can work backwards to find your approximate conception date.
This is the simplest and quickest calculation. It is particularly useful when you already have a confirmed ultrasound-based EDD, which is more accurate than one calculated purely from LMP.
For a standard 28-day cycle, ovulation — and therefore the conception window — occurs around Day 14 of the cycle, counting from Day 1 (the first day of the period). Conception is then:
Not everyone has a 28-day cycle. In fact, among Indian women, cycle length can range widely from 21 to 45 days — and the ovulation window shifts accordingly. The key insight is that the luteal phase (post-ovulation) is almost always 14 days in a normal cycle. It is the follicular phase (pre-ovulation) that varies.
| Cycle Length | Ovulation Day (approx.) | Days to Add to LMP |
|---|---|---|
| 21 days (short) | Day 7 | +7 days |
| 24 days | Day 10 | +10 days |
| 28 days (standard) | Day 14 | +14 days |
| 30 days | Day 16 | +16 days |
| 35 days | Day 21 | +21 days |
| 40 days (long / PCOS) | Day 26 | +26 days |
⚠️ Irregular cycles and PCOS: Women with polycystic ovarian syndrome (PCOS) — which affects approximately 20% of Indian women — often have very irregular cycles with unpredictable ovulation timing. LMP-based conception date calculation is unreliable in this group. Ultrasound dating is significantly more accurate.
Many people believe that conception can only happen on the single day of ovulation. In reality, the fertile window spans 5–7 days around ovulation, because sperm can survive in the female reproductive tract for up to 5 days, while the egg survives for only 12–24 hours after ovulation.
(Shown for a standard 28-day cycle. Shift all days forward or backward proportionally for longer or shorter cycles.)
This fertile window is the reason the conception date is always an estimate — if intercourse occurred on Day 10 but ovulation happened on Day 14, the sperm survived four days in the tract before fertilising the egg. The "conception date" in this scenario is technically Day 14, not Day 10.
IVF is the one scenario where the conception date can be calculated with near-perfect precision — because fertilisation happened in a laboratory at a known time, and the exact age of the embryo at transfer is documented.
Conception Date = Transfer Date − 2 days (the day of egg retrieval and fertilisation)
Conception Date = Transfer Date − 3 days
Conception Date = Transfer Date − 5 days. Most FET cycles use Day 5 blastocysts.
Conception Date = Transfer Date − 6 days. Used for embryos that took longer to develop.
✅ Master formula for IVF: EDD = Transfer Date + (266 − Embryo Age in days). This is verified against ACOG and FOGSI IVF dating guidelines and is the standard used by IVF centres worldwide.
For frozen embryo transfer (FET) cycles, the same formula applies — the embryo's age at the time of freezing is factored into the calculation. Your IVF clinic should document the embryo's precise developmental stage in your records.
An early ultrasound performed between 7 and 13 weeks gestation measures the crown-rump length (CRL) — the length of the embryo/fetus from the top of the head to the base of the spine. CRL is the most accurate method of dating a pregnancy and can place the gestational age within ±3–5 days.
From the gestational age given by ultrasound, the conception date is calculated as:
The 14-day addition accounts for the fact that gestational age is counted from LMP (Day 0), while conception occurred approximately 14 days later. An ultrasound reporting 9 weeks means 9 weeks since LMP — so conception was approximately 7 weeks (9 − 2 weeks) before the scan.
🔊 When your dates change after a scan: If the ultrasound EDD differs from your LMP-based EDD by more than 7 days in the first trimester, the ultrasound date is used as the official EDD. This is standard FOGSI and ACOG practice.
From the moment of conception, development proceeds at a remarkable pace. Here are the key milestones referenced from the conception date (not LMP):
| Weeks from Conception | Gestational Age (from LMP) | Baby Size | Key Milestone |
|---|---|---|---|
| 2 weeks | 4 weeks LMP | 🌱 Poppy seed (~1 mm) | Implantation complete; hCG begins rising — pregnancy test may turn positive |
| 4 weeks | 6 weeks LMP | 🫘 Lentil (~4 mm) | Heartbeat detectable on transvaginal ultrasound; neural tube closing |
| 6 weeks | 8 weeks LMP | 🫘 Kidney bean (~1.6 cm) | All major organs forming; facial features beginning; limb buds present |
| 8 weeks | 10 weeks LMP | 🍓 Strawberry (~3.1 cm) | Embryo officially becomes a fetus; external genitalia beginning to differentiate |
| 10 weeks | 12 weeks LMP | 🍋 Lime (~5.4 cm, 14g) | NT scan week — nuchal translucency measurement for Down syndrome risk |
| 14 weeks | 16 weeks LMP | 🥑 Avocado (~11.6 cm, 100g) | Gender may be visible; quickening (first movements felt) approaches |
| 18 weeks | 20 weeks LMP | 🍌 Banana (~25.6 cm, 300g) | Anomaly / TIFFA scan — detailed anatomy assessment. Halfway point. |
| 22 weeks | 24 weeks LMP | 🌽 Corn (~30 cm, 600g) | Viability threshold; GDM screening (75g OGTT) between 24–28 weeks |
| 26 weeks | 28 weeks LMP | 🍆 Brinjal (~37.6 cm, 1 kg) | Third trimester begins; Anti-D for Rh-negative mothers |
| 30 weeks | 32 weeks LMP | 🥦 Large squash (~42.4 cm, 1.7 kg) | Growth scan + colour Doppler; check fetal presentation |
| 38 weeks | 40 weeks LMP | 🍉 Watermelon (~51 cm, 3.4 kg) | Full term — estimated due date reached |
After fertilisation, the embryo travels down the fallopian tube and implants in the uterine wall approximately 6–12 days after conception. At this point, the embryo begins producing human chorionic gonadotrophin (hCG) — the hormone detected by pregnancy tests. Symptoms, if they appear, typically begin around 4–6 weeks gestational age (2–4 weeks after conception):
✅ When to take a pregnancy test: Home urine pregnancy tests detect hCG reliably from the first day of a missed period — approximately 14 days after conception. Testing earlier may give a false negative even if conception has occurred, because hCG levels are not yet high enough to detect. For the most accurate result, test with the first morning urine (most concentrated).
Several factors make conception timing calculations particularly important to individualise for Indian women:
🌸 Calculate your conception date now: Use the RxMedCalc Conception Date Calculator → — supports all 4 methods (due date, LMP, IVF, ultrasound) with week-by-week fetal development, Indian antenatal scan schedule, and results you can share via WhatsApp.
This article is for informational purposes based on FOGSI, ACOG, and WHO obstetric dating guidelines. Conception date is an estimate in naturally conceived pregnancies. Always confirm gestational age and due date with your registered obstetrician or gynaecologist through clinical assessment and ultrasound.
Built by an MBBS, AFIH Certified Physician in Punjab, India | RxMedCalc.com