Conception & IVF Due Date Calculator
Find out when you most likely conceived, or get your due date from a conception, IUI or IVF transfer date, with the key dates ahead.
A longer cycle moves ovulation, and so conception, later by the same number of days. The luteal phase is the time from ovulation to your next period, usually 14 days; leave it unless you track ovulation.
Works for "when was I conceived?" too. Leave 40 weeks if you don't know; a baby born a week early was conceived a week later than the birth date suggests.
Day 5 and 6 embryos are blastocysts. A frozen or donor-egg transfer uses the same maths: count from the embryo's age on transfer day.
Eggs are fertilised on retrieval day (embryo day 0), so this works for fresh transfers of any embryo age.
Want more? Our due date calculator shows weeks pregnant and dating-scan rules, and the ovulation calculator plans your fertile days for the months ahead.
How your dates are worked out
Conception is about 2 weeks after day 0 and 38 weeks (266 days) before the due date.
- 1
Start at due date
17 May 2027
is day 280 - 2
Count back 38 weeks
− 266 days
= 24 August 2026 - 3
Fertile window
6 days up to it
19 – 24 August - 4
How far along
70 days ÷ 7
= 10w 0d
baby's age 8w 0d
Read more: the formula, a worked example, IVF dating and notes
Pregnancy is counted from "day 0", the first day of the last period, or the date that stands in for it. Conception, which is when the egg is fertilised, is counted as day 14, and the due date is day 280 (40 weeks). So conception is always 266 days (38 weeks) before the due date. After IVF the embryo's age is known exactly, so the transfer date pins down every other date.
The formula
conception = due date − 266 days
conception = last period + cycle length − luteal phase (14)
due date = conception, IUI or egg retrieval + 266 days
due date = IVF transfer + 266 − embryo age in days
fertile window = the 5 days before conception + that day
weeks pregnant = days since day 0 ÷ 7; baby's age = that − 14 days
| You know | Due date is | Pregnancy length that day |
|---|---|---|
| Conception or ovulation date | + 266 days | 2w 0d |
| IUI date | + 266 days | 2w 0d |
| Egg retrieval date | + 266 days | 2w 0d |
| IVF transfer, day 2 embryo | + 264 days | 2w 2d |
| IVF transfer, day 3 embryo | + 263 days | 2w 3d |
| IVF transfer, day 5 blastocyst | + 261 days | 2w 5d |
| IVF transfer, day 6 blastocyst | + 260 days | 2w 6d |
| First day of last period (28-day cycle) | + 280 days | 0w 0d |
Worked example
Say your due date is 8 October 2026. Count back 266 days: you most likely conceived on 15 January 2026. The fertile window is the 5 days before that plus the day itself, 10 to 15 January, so the sex that led to the pregnancy was most likely in those 6 days. Day 0 was 15 January − 14 days = 1 January 2026, so implantation most likely happened between 21 and 27 January (6 to 12 days after conception) and a pregnancy test would be reliable from 29 January (4 weeks 0 days).
On 1 April 2026 you'd be 90 days past day 0: 90 ÷ 7 = 12 weeks 6 days pregnant, and your baby's own age would be 90 − 14 = 76 days, or 10 weeks 6 days. The second trimester starts on 9 April (14 weeks) and the third on 16 July (28 weeks). The same dates come from a last period on 1 January with a 28-day cycle (1 January + 28 − 14 = 15 January), a day-5 IVF transfer on 20 January (20 January + 261 days = 8 October), a day-3 transfer on 18 January, or an egg retrieval on 15 January.
IVF, frozen transfers and donor eggs
With IVF the eggs are fertilised on retrieval day, which counts as embryo day 0. A day-5 embryo is 5 days old when it goes in, so on transfer day you count as 2 weeks 5 days pregnant. Frozen embryo transfers (FET) and donor-egg cycles use the same maths: what matters is the embryo's age on transfer day, not when it was frozen or the age of the person carrying the pregnancy. The American College of Obstetricians and Gynecologists (Committee Opinion 700) says IVF due dates should use the embryo's age and the transfer date.
Gestational age and embryo age
| Weeks pregnant (gestational age) | Baby's age (embryo / fetal age) |
|---|---|
| 2w 0d (conception) | 0w 0d |
| 4w 0d (missed period) | 2w 0d |
| 6w 0d | 4w 0d |
| 8w 0d | 6w 0d |
| 12w 0d | 10w 0d |
| 20w 0d | 18w 0d |
| 40w 0d (due date) | 38w 0d |
Doctors, midwives and scans almost always use gestational age. Most fertility clinics use it too, so a clinic's "6-week scan" means 6 weeks counted from day 0.
Please note
- These are estimates. Only about 1 in 20 babies arrive on the due date, and an early dating scan (usually between 8 and 14 weeks) may change your dates. Your midwife or doctor will use the scan date from then on.
- Ovulation doesn't always happen on day 14. Stress, illness, travel and conditions such as PCOS can move it, so a date from your last period could be several days out.
- "When was I conceived?" from a birth date assumes a 40-week pregnancy unless you enter the weeks at birth.
Not medical advice. For information only: please speak to your doctor, fertility clinic or midwife about your pregnancy and care. Checked on 11 October 2026.
Common questions
How do I work out when I conceived?
Take your due date and count back 266 days (38 weeks). That's the most likely day you conceived, which is usually the day you ovulated. If you only know your last period, conception is usually about 14 days before your next period was due: day 15 of a 28-day cycle.
Can I know the exact day I conceived?
Usually not. Sperm can live for up to 5 days, so the sex that led to the pregnancy could have been up to 5 days before ovulation. The calculator shows the most likely conception day and the 6-day fertile window before it. Dates from an early scan are the most accurate.
How is an IVF due date calculated?
The embryo's age is known exactly, so count from the transfer date. Add 263 days for a day-3 embryo, 261 days for a day-5 blastocyst or 260 days for a day-6 embryo. From the egg retrieval date, add 266 days. A frozen transfer uses the same maths, based on the embryo's age at transfer.
How many weeks pregnant am I after an embryo transfer?
Pregnancy is counted as if you had a period 14 days before the eggs were fertilised. So on the day of a day-5 transfer you count as 2 weeks 5 days pregnant, and on a day-3 transfer as 2 weeks 3 days. Your baby's own age is always 2 weeks less than this.
What is the difference between gestational age and embryo age?
Gestational age is the usual "weeks pregnant" number, counted from day 0, about 2 weeks before conception. Embryo or fetal age is the baby's real age since conception, so it is 14 days less. A due date at 40 weeks is 38 weeks after conception.
When can I take a pregnancy test after conception?
A home test is most reliable from the day your period is due, about 14 days after conception, which is 4 weeks 0 days pregnant. After IVF, follow your clinic's test day. Implantation usually happens 6 to 12 days after ovulation, and the pregnancy hormone rises after that.
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