How accurate are due dates?

Only about 4% of births happen on the estimated date, though roughly 60% fall within a week of it. Full term is 37 to 42 weeks, so a healthy birth can be several weeks either side. A first-trimester ultrasound is the most accurate dating method available.

How is a due date calculated after IVF?

From the transfer date, because fertilisation is known exactly. Add 266 days and subtract the embryo's age at transfer: 263 days for a day-3 embryo, 261 for a day-5 blastocyst. IVF dating is the most precise of all methods, and clinicians generally keep it even if a later scan suggests otherwise.

Can I calculate a due date from the conception date?

Yes — add 266 days (38 weeks) to the conception date, since conception typically occurs about two weeks after the last period began. Most people know their period date more reliably than their conception date, which is why the standard method uses it.

Why does my ultrasound date differ from my period-based date?

Because the period-based calculation assumes ovulation on day 14 of a 28-day cycle, which is not true for everyone. Cycle length and ovulation timing vary. Early embryonic growth is far more consistent, so a first-trimester ultrasound is more accurate and clinicians usually adopt that date when the two disagree materially.

Can my due date change?

Yes, most often after the first ultrasound, if it disagrees with the period-based estimate by more than about a week. Once a date is set from an early scan it is normally kept for the rest of the pregnancy; later scans measure growth rather than re-date it.

What does gestational age mean?

How far along the pregnancy is, measured from the first day of your last period and expressed as weeks and days — for example 24w 3d. It runs about two weeks ahead of the embryo's actual age because it starts before conception occurred.

Due Date Calculator — Pregnancy Due Date

Estimate your due date from the first day of your last period, a conception date, an IVF transfer date, or a dated ultrasound — with today's gestational age, the trimester boundaries, and the early-, full- and post-term dates.

How due dates are estimated

The standard method is Naegele's rule: add 280 days — 40 weeks — to the first day of your last menstrual period. This calculator applies that rule and adjusts for cycle length, since the rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, ovulation and therefore conception shift accordingly.

Because dating starts from your last period rather than conception, gestational age is roughly two weeks ahead of the embryo's actual age. This is why you are considered four weeks pregnant about two weeks after conception, which is a common source of confusion.

Due date by dating method
You know	Calculation	Typical accuracy
First day of last period	+ 280 days, adjusted for cycle length	About ± 2 weeks; depends on regular cycles and recall
Conception date	+ 266 days (38 weeks)	Only as good as the conception date itself
IVF transfer date	+ 266 days, less the embryo age (263 for day 3, 261 for day 5)	Most precise — fertilisation date is known
First-trimester ultrasound	Scan date − gestational age at scan, + 280 days	± 5 to 7 days; the clinical reference
Second-trimester ultrasound	As above	± 10 to 14 days
Third-trimester ultrasound	As above	± 3 weeks; not used to change an established date

How accurate is a due date?

Less precise than the specific date suggests. Only about 4% of births occur exactly on the estimated due date, and roughly 60% occur within a week either side. Full term spans 37 to 42 weeks, so a healthy delivery can be three weeks early or two weeks late.

An ultrasound in the first trimester is the most accurate dating method, typically within five to seven days, because early embryonic growth is highly consistent across pregnancies. If an early ultrasound date differs materially from the last-period estimate, clinicians generally adopt the ultrasound date. Later ultrasounds are less reliable for dating, since growth rates diverge more as pregnancy progresses.

How term is defined
Stage	Gestational age	Notes
Preterm	before 37 weeks 0 days	Earlier birth carries more risk; care escalates the earlier it is
Early term	37w 0d – 38w 6d	Healthy, but outcomes are slightly better a fortnight later
Full term	39w 0d – 40w 6d	The window the due date sits in
Late term	41w 0d – 41w 6d	Monitoring increases; induction is often offered
Post-term	42 weeks 0 days and beyond	Induction is usually recommended

The three trimesters

The first trimester runs from week 1 through week 13. Major organ systems form during this period, and most miscarriages that occur do so here. The second trimester, weeks 14 through 27, is when many people feel best; the anatomy scan usually happens around weeks 18 to 22, and movement typically becomes noticeable between weeks 18 and 25.

The third trimester covers week 28 to birth. Weight gain accelerates, and viability outside the womb improves substantially through this period. Weeks 37 to 38 are considered early term, 39 to 40 full term, 41 late term, and beyond 42 post-term, when induction is usually discussed.

Trimesters and common appointments
Trimester	Weeks	What usually happens
First	1 – 13	Booking visit (8–12 wk); dating scan; screening blood tests and NIPT (from 10 wk); nuchal scan (11–14 wk)
Second	14 – 27	Anatomy scan (18–22 wk); movements felt (18–25 wk); glucose test (24–28 wk)
Third	28 – birth	Rh antibody check and Tdap (27–36 wk); group B strep swab (35–37 wk); weekly visits from 36 wk

Milestones by week

Timings vary by country and clinic, but the sequence below is the common pattern. Use it to place the dates the calculator gives you, not as a schedule — your care team's plan takes precedence.

Typical pregnancy milestones
Week	Milestone
4	Missed period; home test turns positive
6	Heartbeat often visible on early ultrasound
8 – 12	First prenatal visit and dating scan
10 – 13	Cell-free DNA (NIPT) and first screening bloods
11 – 14	Nuchal translucency scan
18 – 22	Anatomy scan; sex can usually be determined
18 – 25	Movements first felt
24	Threshold of viability outside the womb
24 – 28	Gestational diabetes glucose test
27 – 36	Tdap vaccination; Rh antibody screen for Rh-negative mothers
28	Third trimester begins
35 – 37	Group B streptococcus swab
37	Early term
39	Full term
40	Estimated due date
41 – 42	Late term; induction discussed

Last reviewed September 9, 2026. Results are estimates for informational purposes only.

Pregnancy Due Date Calculator

How this was calculated

How due dates are estimated

How accurate is a due date?

The three trimesters

Milestones by week

Frequently asked questions