First trimester
Implantation through to the end of organ formation. The riskiest weeks, and the ones with the least to show for them. Dating scan at six to eight weeks, screening at eleven to fourteen.
Tools & Calculators
Forty weeks,
Forty weeks,
counted properly.
Most due date calculators ask for one thing: the first day of your last period. That answer assumes you ovulated on day fourteen, and it is the least accurate input available. If you conceived through IVF we can date the pregnancy to the day.
The short answer
A due date is 280 days from the first day of your last period, or 266 days from conception. After IVF it is exact: 261 days from a day-5 transfer, 263 from a day-3 transfer, 266 from an egg retrieval. It is a median, not a deadline. Only about one baby in twenty arrives on the date itself, and anything from thirty seven weeks is term.
Day-5 transferTransfer date plus 261 days, accurate to the day
Last periodPlus 280 days, corrected for your cycle length
On the date itselfAbout 1 in 20 births
Term37 weeks onward, most births by 41
Work out your date
Pick the most accurate thing you know. The methods are listed best first, which is the opposite of how most calculators order them.
Clinical reference: ACOG Committee Opinion No. 700, Methods for Estimating the Due Date.
When babies actually arrive
Spontaneous onset, singleton pregnancy, by completed week. The due date is the middle of this distribution, which is the single most useful thing to understand about it.
6%
34 to 36
Preterm
8%
37
Early term
15%
38
Early term
26%
39
Full term
23%
40
Full term
16%
41
Late term
6%
42+
Post term
1 in 20Born on the due date
6 in 10Between 37 and 40 weeks
5 weeksThe normal spread of term
Figures are for spontaneous onset and exclude planned caesareans and inductions, which shift the picture toward 39 weeks. Your own likelihood is shaped by parity, previous deliveries and whether this is a multiple pregnancy.
Your timeline, dated
Every appointment placed against your own estimate. The rail fills as you scroll, and anything already behind you is marked.
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7w
Scan
Dating and viability scan
Confirms the pregnancy is in the uterus, that there is a heartbeat, and how many there are. After IVF this confirms rather than establishes the dates.
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12w
Scan and bloods
First trimester screening
Nuchal translucency measurement with the combined blood test, between eleven and thirteen weeks plus six days. NIPT can be done from ten weeks if you want it.
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16w
Visit
Second trimester review
Blood pressure, urine, growth and a conversation about how you actually feel. Anti-D if you are Rh negative and it is indicated.
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20w
Scan
Anomaly scan
The detailed anatomy scan, between eighteen and twenty two weeks. Heart, brain, spine, kidneys, limbs, and the position of the placenta.
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26w
Bloods
Glucose tolerance test
Screening for gestational diabetes between twenty four and twenty eight weeks, earlier if there are risk factors such as PCOS or a raised BMI.
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32w
Scan
Growth scan
Estimated fetal weight, liquor volume and Dopplers. Visits move to every two weeks from here.
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36w
Visit
Presentation and planning
Confirms the baby is head down, and the point at which the birth plan stops being hypothetical. Group B strep swab if your protocol takes one.
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37w
Milestone
Term begins
From here the pregnancy is term. Labour starting now is not early, and roughly six in ten babies arrive between now and the due date.
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40w
Milestone
Estimated due date
The median, not a deadline. Around one baby in twenty arrives on this exact day.
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41w
Visit
Post-dates monitoring
Twice-weekly monitoring, and a conversation about induction, usually offered between forty one and forty two weeks.
Trimester by trimester
Where you are now is marked. Dates come from the estimate above.
Second trimester
Usually the easiest stretch. Sickness settles, energy returns, movements begin around eighteen to twenty two weeks, and the anomaly scan checks anatomy in detail.
Third trimester
Growth and position. Visits get closer together, a growth scan around thirty two weeks, and from thirty seven weeks the pregnancy is term and labour can begin at any point.
Why the method matters
Four things that separate a date you can plan around from one that quietly moves.
IVF dates are not estimates
When fertilisation happened in a laboratory on a known morning, gestational age is a fact rather than an inference. Nothing else in obstetrics is dated this precisely.
A period date assumes day 14
Naegele's rule is built on a 28 day cycle with ovulation exactly halfway. If your cycle runs to 34 days, an unadjusted calculator puts your due date most of a week early.
Early scans beat late ones
A crown-rump length before fourteen weeks dates a pregnancy to within about five days. The same measurement in the third trimester can be out by three weeks, because babies grow at different rates.
The date should not keep moving
Once dating is established from the earliest reliable source it stays fixed. Re-dating a pregnancy on a late growth scan is a common error and it distorts everything measured against it.
Due date questions
How it is worked out How is a due date calculated?
The standard method is Naegele's rule: 280 days, or forty weeks, from the first day of your last menstrual period. It assumes a 28 day cycle with ovulation on day 14, so a good calculator adjusts it for your actual cycle length. If conception, an insemination, an egg retrieval or an embryo transfer date is known, counting forward from that is more accurate, because it removes the assumption about when you ovulated.
After IVF How is the due date calculated after IVF?
From the transfer date, adjusted for how old the embryo was. A day-5 blastocyst transfer gives a due date 261 days later; a day-3 transfer, 263 days later; an egg retrieval, 266 days later. Because fertilisation happened on a known day in the laboratory, an IVF due date is exact rather than estimated, and it should not be changed by a later scan.
After IVF Is an IVF due date more accurate than a scan?
Yes. A first trimester crown-rump length dates a pregnancy to within about five days, which is very good. An embryo transfer date is accurate to the day, which is better. Where the two disagree slightly, established practice is to keep the IVF dating, because it is derived from a known event rather than a measurement with a margin.
How it is worked out Does my cycle length change my due date?
Yes, if the date is being worked out from your last period. Naegele's rule assumes ovulation on day 14, so on a 35 day cycle you ovulated around day 21 and your due date is roughly a week later than an unadjusted calculator will tell you. This estimator applies that correction. If the date is coming from a transfer, a retrieval or a known conception, cycle length is irrelevant.
Arriving What are the chances of giving birth on my due date?
About one in twenty. The due date is the median of a distribution, not a deadline: roughly a quarter of babies arrive in week 39, another quarter in week 40, and about one in six in week 41. Anything from thirty seven weeks is term, and the great majority of spontaneous births fall between thirty seven and forty two weeks.
Arriving What does full term actually mean?
Thirty seven weeks completed is early term, thirty nine to forty weeks is full term, forty one weeks is late term, and beyond forty two weeks is post term. The distinction matters because outcomes at thirty nine to forty weeks are measurably better than at thirty seven, which is why elective delivery is not usually offered before thirty nine weeks without a reason.
How it is worked out Can my due date change?
It can be revised once, if the first accurate scan disagrees materially with the date worked out from your period, which is common because period dates are unreliable. Once dating is established from the earliest reliable source it should then stay fixed. Re-dating on a third trimester growth scan is a mistake: babies differ in size by then, and moving the date distorts every growth measurement made against it.
Arriving How many weeks pregnant am I?
Gestational age is counted from the first day of your last period, not from conception, which is why you are already considered two weeks pregnant at the moment of conception. This estimator shows both: weeks of gestation, which is what your clinician uses, and days since the pregnancy actually began.
Nothing in that category. Try another.
Next step
A date is the start of the plan, not the plan.
Antenatal care at One World Fertility runs from the first scan through to delivery with the same team that got you here. Bring your dates and we will build the schedule around them.