How is the conception date calculated?
Conception usually happens within a day of ovulation, which in a typical 28-day cycle is about 2 weeks after the first day of your last period. Doctors date pregnancy from the last period, so conception is usually around 2 weeks after your LMP, or 266 days (38 weeks) before your due date.
Because sperm can survive up to 5 days, the sex that led to pregnancy may have happened up to 5 days before conception. That is why we show a window rather than a single day.
Conception date from a birth date
If you know your baby’s birth date, we count back 266 days — assuming a full-term birth. A full-term baby can arrive anywhere between 37 and 42 weeks, so the possible conception range is about 5 weeks wide. If your baby was born early, conception was closer to the birth date.
| You know |
Conception ≈ |
| First day of last period (28-day cycle) |
LMP + 14 days |
| Due date |
Due date − 266 days |
| Birth date (full term) |
Birth date − 266 days (range −280 to −245) |
A calculator cannot prove the exact day of conception or paternity — only a DNA test can do that.
Frequently asked questions
How accurate is a conception date calculator?
It gives an estimate within a few days if your cycles are regular and your due date is from an early ultrasound. With irregular cycles or an unknown last period, the range is wider.
Can I find out the exact day I got pregnant?
Not exactly. Ovulation can vary, and sperm can live for up to 5 days, so the sex that led to pregnancy could have been up to 5 days before ovulation.
How many weeks pregnant am I at conception?
About 2 weeks, because pregnancy is counted from the first day of your last period.
Sources: ACOG: Methods for estimating the due date · Timing of intercourse and conception (NEJM 1995)
Reviewed for accuracy against the sources above. This page is general information, not medical advice — talk to your doctor or a registered dietitian about your own needs.
More free health tools
🌸Ovulation CalculatorFind your fertile window and best days to conceive.Open tool →👶Pregnancy Due Date CalculatorFree pregnancy due date calculator — use your last period, conception date or IVF…Open tool →🤰Pregnancy Week CalculatorFind out exactly how many weeks and days pregnant you are, your trimester and…Open tool →📅Period CalculatorFree period calculator: enter your last period date and cycle length to predict your…Open tool →🧪When to Take a Pregnancy TestFind the best day to take a home pregnancy test for an accurate result…Open tool →🌱Implantation CalculatorEstimate when implantation may happen — from your last period, ovulation date or IVF…Open tool →⚖️Pregnancy Weight Gain CalculatorCheck if your pregnancy weight gain is on track for your week — based…Open tool →🧫IVF Due Date CalculatorCalculate your IVF due date from a day-3, day-5 or day-6 embryo transfer (fresh…Open tool →📈Baby Growth Percentile CalculatorFree WHO baby growth percentile calculator for 0–5 years — weight, length/height and head…Open tool →💉Baby Vaccination Schedule CalculatorEnter your baby’s birth date to get every vaccine due date — India IAP…Open tool →🍼Breast Milk & Formula CalculatorFind how much breast milk or formula your baby needs per feed and per…Open tool →👣Baby Kick CounterFree online kick counter for pregnancy — tap to count your baby’s movements, time…Open tool →⏱️Contraction TimerFree contraction timer — tap to time how long contractions last and how far…Open tool →🔥Calorie Calculator (TDEE & BMR)Free calorie calculator — find your BMR and TDEE and how many calories a…Open tool →🎯Ideal Weight CalculatorFind your ideal weight for your height — healthy BMI range (including Asian cut-offs)…Open tool →📏Body Fat CalculatorEstimate your body fat percentage with a tape measure using the US Navy method…Open tool →📐Waist-to-Hip Ratio CalculatorCalculate your waist-to-hip ratio and waist-to-height ratio and see what they mean for heart…Open tool →🧮Glycemic Index CheckerSearch the glycemic index (GI) and glycemic load of 60+ Indian and global foods…Open tool →🧮Macro CalculatorFree macro calculator — your daily calories split into grams of carbs, protein and…Open tool →🧮Iron Requirement CalculatorFind how much iron you need each day by age, sex, pregnancy and diet…Open tool →🧮Child & Teen BMI CalculatorFree BMI calculator for children and teens aged 2–19 — BMI, percentile and WHO…Open tool →🧮Asian BMI CalculatorFree BMI calculator with Asian and Indian cut-offs — normal 18.5–22.9, overweight 23–24.9, obese…Open tool →🧮Steps to Calories CalculatorConvert steps into distance and calories burned based on your weight, height and walking…Open tool →🧮Indian Diabetes Risk ScoreCheck your type 2 diabetes risk in one minute with the Indian Diabetes Risk…Open tool →🧮PCOS Symptom CheckerFree PCOS symptom checker based on the Rotterdam criteria doctors use — periods, excess…Open tool →🧮Thyroid Symptom CheckerFree thyroid symptom checker — tick your symptoms to see if they point towards…Open tool →🧮Anaemia Symptom CheckerFree anaemia (anemia) symptom checker — tick your symptoms and risk factors to see…Open tool →🧮Baby Weight Chart by AgeNormal baby weight by age for boys and girls from birth to 5 years…Open tool →🧮Child Height PredictorPredict your child’s adult height from both parents’ heights using the mid-parental height method…Open tool →