Understanding trimester stages
Clinicians divide pregnancy into three trimesters that correspond to major developmental milestones. The boundaries below reflect widely used definitions and the gestational-age offsets applied by this calculator.
| Trimester | Gestational age range | Key developments |
|---|---|---|
| First trimester | Weeks 1 – 13 (days 0 – 97) | Embryonic organ formation; highest period of miscarriage risk; first-trimester screening window (weeks 10–13). |
| Second trimester | Weeks 14 – 27 (days 98 – 195) | Fetal growth accelerates; anatomy scan typically offered around weeks 18–22; risk of miscarriage falls substantially. |
| Third trimester | Weeks 28 – 40+ (days 196 onward) | Rapid brain and lung maturation; fetal movement monitoring; birth planning; viability well-established by this stage. |
- Only approximately 4% of births occur on the calculated EDD. A normal spontaneous labour can occur anywhere from 37 weeks 0 days to 42 weeks 0 days of gestational age (ACOG definition of term).
- First-trimester ultrasound is the most accurate dating method. ACOG Committee Opinion 700 (2017) recommends revising the EDD when the ultrasound and LMP-based dates differ by more than 7 days in the first trimester.
- IVF dating offsets (263 days for day-3, 261 days for day-5 transfers) follow American Society for Reproductive Medicine (ASRM) practice. Your fertility clinic may apply slightly different conventions — confirm with your care team.
- This calculator does not account for multiple pregnancies (twins, triplets), which are typically managed to earlier delivery milestones. For multiple pregnancies, all estimates should be interpreted with your obstetric care team.
- This tool produces estimates only and does not constitute medical advice. Always confirm your due date with a qualified obstetrician or midwife.
What is a pregnancy due date?
An estimated due date (EDD) is the calendar date on which a pregnancy is expected to reach 40 weeks of gestational age. The American College of Obstetricians and Gynecologists (ACOG) uses gestational age — counted from the first day of the last menstrual period — as the standard measure of pregnancy duration. A full-term pregnancy spans 39 weeks 0 days to 40 weeks 6 days by current ACOG terminology.
Gestational age differs from embryonic age (time since fertilisation). Because ovulation typically occurs around cycle day 14, gestational age is approximately two weeks longer than embryonic age. All EDD methods described on this page report gestational age in line with ACOG clinical practice.
First-trimester ultrasound measurement of the fetal crown-rump length is widely considered the most accurate dating method when performed between 8 and 13 weeks 6 days. The American College of Obstetricians and Gynecologists recommends that if the EDD assigned by LMP and the ultrasound EDD differ by more than 7 days in the first trimester, the ultrasound date should be used (ACOG Committee Opinion 700, 2017). Calendar-based methods such as this calculator are useful planning estimates but should always be confirmed by your healthcare provider.
How to use this due date calculator
- Select your dating method. Choose 'Last menstrual period' if you know when your period started, 'Conception date' if you tracked ovulation precisely, or the appropriate IVF transfer option if your pregnancy began with in vitro fertilisation.
- Enter the relevant date. For LMP, enter the first day of your most recent period before pregnancy. For conception, enter the date of confirmed ovulation or insemination. For IVF, enter the date of the embryo transfer procedure.
- For the LMP method, enter your average cycle length (20–45 days). The default is 28 days; a longer or shorter cycle shifts the EDD accordingly.
- Your estimated due date, current gestational age (if applicable), and approximate trimester boundaries appear immediately.
The dating formulas explained
Naegele's Rule, derived from the LMP method, forms the basis of standard obstetric dating. The original rule adds 280 days (40 weeks) to the first day of the LMP. This calculator adjusts for non-28-day cycles by adding the difference between the entered cycle length and 28 days to the base 280-day offset.
For a known conception date, the calculator adds 266 days (38 weeks), reflecting the average duration from fertilisation to delivery. For IVF pregnancies, ASRM conventions specify 263 days from a day-3 embryo transfer and 261 days from a day-5 blastocyst transfer, accounting for the embryonic age already established at transfer.
Trimester boundaries used here follow common clinical conventions: the second trimester begins at 14 weeks 0 days of gestational age (approximately 98 days after the gestational start date), and the third trimester begins at 28 weeks 0 days (approximately 196 days). Some sources define the third trimester from 27 or 28 weeks; the boundary is not universally standardised.
Frequently asked questions
How accurate is a due date calculator?
Calendar-based due date calculators are useful planning estimates, but they carry inherent uncertainty. Research shows that only about 4% of births occur on the calculated EDD. First-trimester ultrasound measurement of the crown-rump length is more accurate than calendar methods and is the reference standard recommended by ACOG (Committee Opinion 700, 2017). Your obstetrician or midwife will confirm and may adjust your EDD after an ultrasound scan.
What is Naegele's Rule?
Naegele's Rule is the traditional formula for estimating the due date from the last menstrual period. It adds 280 days (40 weeks) to the first day of the LMP. The rule assumes a 28-day cycle with ovulation on day 14; this calculator adjusts the offset for cycles that differ from 28 days. Naegele's Rule was described by the German obstetrician Franz Karl Naegele in the early 19th century and remains the starting point for clinical dating worldwide.
Why is gestational age counted from the LMP, not from conception?
Gestational age is counted from the LMP because the exact date of ovulation and fertilisation is often unknown, whereas the first day of the last period is usually remembered. Because ovulation typically occurs around day 14 of a 28-day cycle, gestational age is approximately two weeks longer than embryonic age (time since fertilisation). ACOG, the WHO and most obstetric guidelines use gestational age as the standard clinical measure.
How does cycle length affect my due date?
The standard Naegele calculation assumes a 28-day cycle. If your cycles are consistently shorter or longer, ovulation and conception occur earlier or later relative to your LMP. This calculator adjusts the EDD by the difference between your cycle length and 28 days: a 35-day cycle adds 7 days to the LMP-based EDD, while a 21-day cycle subtracts 7 days. Cycle length has no effect when you select the conception or IVF methods, because those methods are anchored to embryonic age directly.
What is the difference between IVF day-3 and day-5 due dates?
In IVF, embryos are cultivated in the laboratory for either 3 days (cleavage-stage) or 5 days (blastocyst stage) before transfer. Because the embryo is already 3 or 5 days old at transfer, the gestational age at transfer is higher for day-5 embryos. Following ASRM conventions, this calculator adds 263 days from a day-3 transfer and 261 days from a day-5 transfer. The resulting EDD is typically similar for both methods; the small difference reflects the two extra days of in vitro development.
What does 'term pregnancy' mean?
The American College of Obstetricians and Gynecologists defines a full-term pregnancy as 39 weeks 0 days to 40 weeks 6 days of gestational age. Early-term is 37–38 weeks 6 days, late-term is 41–41 weeks 6 days, and post-term is 42 weeks or beyond. Preterm birth is defined as delivery before 37 completed weeks. These categories guide clinical decisions about timing of delivery.
Can I use this calculator for a multiple pregnancy (twins)?
The calculator will produce an EDD based on the same formulas, but multiple pregnancies are typically managed to different delivery milestones than singleton pregnancies. Uncomplicated dichorionic-diamniotic (DCDA) twins are commonly delivered around 38 weeks, and monochorionic pregnancies earlier still. For any multiple pregnancy, the EDD estimate from this tool should be interpreted alongside close monitoring by a maternal-fetal medicine specialist.
References
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol 2017; 129(5): e150–e154.
- American Society for Reproductive Medicine (ASRM). Guidance on the limits to the number of embryos to transfer: a committee opinion. Fertil Steril 2017; 107(4): 901–903.
- American College of Obstetricians and Gynecologists. Definition of Term Pregnancy. Committee Opinion No. 579. Obstet Gynecol 2013; 122(5): 1139–1140.
- Naegele FC. Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtes. 1812. (Historical source of Naegele's Rule.)
- Savitz DA et al. Spontaneous preterm birth time trends in the United States, 1989–2015. Am J Epidemiol 2018; 187(12): 2537–2545.