Cycle length adjusts the estimated due date and conception date, while completed pregnancy weeks are still counted from the LMP date.
Result
18w 0d
154 days until the estimated due date.
Trimester
Second trimester
Estimated weeks remaining
22 weeks
Pregnancy progress
45.0%
Approx fetal age
16w 0d
Estimated due date
23 Oct 2026
Estimated conception
30 Jan 2026
Dating basis
Last menstrual period: 16 Jan 2026
Total completed days since LMP-equivalent start: 126
Planning milestones
End of first trimester: 23 Apr 2026
End of second trimester: 30 Jul 2026
Typical anatomy-scan window: 22 May 2026 - 19 Jun 2026
37-week mark: 2 Oct 2026
39-week mark: 16 Oct 2026
41-week mark: 30 Oct 2026
42-week mark: 6 Nov 2026
Confirm uncertain dates If your last period is uncertain, cycles are irregular, symptoms are concerning, or an ultrasound gives a different estimate, use your clinician's dating assessment for medical decisions.
Pregnancy weeks, gestational age, and due-date math explained
A pregnancy weeks calculator estimates how far along a pregnancy is in weeks and days. This page also explains the main assumptions behind the pregnancy weeks, gestational age, and due-date math result, highlights the supporting figures shown by the calculator, and helps the reader use the estimate without overstating what a quick online tool can prove.
What this pregnancy weeks calculator is measuring
In obstetrics, pregnancy length is usually described as gestational age, not fetal age. Gestational age is counted from the first day of the last menstrual period, even though conception usually happens about two weeks later. That is why a pregnancy weeks calculator can say someone is 8 weeks pregnant even though the embryo or fetus has existed for fewer than 8 weeks.
This calculator gives a quick estimate in weeks and days because that is how pregnancy milestones, scans, trimesters, and many care decisions are commonly described. It is useful for people asking how many weeks pregnant they are, how many weeks remain until the due date, or what trimester they are in right now.
The date formulas used by the calculator
The live tool supports two entry points. In LMP mode it starts from the first day of the last period and adds 280 days to estimate the due date. In due-date mode it works backward by 280 days to estimate the LMP date. It then calculates current gestational age by counting the days between today and the estimated LMP date.
The calculator also shows an estimated conception date by adding 14 days to the LMP date. That is a standard approximation for a 28-day cycle and is useful for orientation, but real ovulation and conception timing can vary.
Estimated due date = LMP + 280 days
This is the standard 40-week obstetric estimate used by many pregnancy and due-date calculators.
Estimated LMP = due date − 280 days
If the due date is already known, the calculator works backward to estimate the cycle start date used for gestational dating.
Gestational age = today − estimated LMP
The result is shown as completed weeks plus remaining days, for example 18 weeks and 3 days.
Why the estimate can change during pregnancy
A date-based pregnancy weeks calculator is convenient, but clinical dating is not always fixed by the calendar alone. If the last period date is uncertain, cycles are irregular, or conception happened earlier or later than expected, the estimate can shift. First-trimester ultrasound is generally the most accurate way to establish or confirm gestational age when there is uncertainty.
That is why an app or free online pregnancy calculator should be treated as a planning reference rather than as the final clinical word. A clinician may revise the estimated due date when scan findings and menstrual dating differ enough to justify a better obstetric estimate.
LMP-based dating works best when the first day of the last period is known confidently.
A due date is an estimate, not a guarantee of the birth date.
Most births do not happen exactly on the estimated due date.
Ultrasound can replace or refine date-based estimates when the dates are uncertain.
How to interpret weeks, trimesters, and weeks remaining
This calculator labels the first trimester through 13 completed weeks, the second trimester through 27 completed weeks, and the third trimester from 28 weeks onward. It also estimates weeks remaining by counting forward to the due date. That makes it useful as a pregnancy weeks calculator, date countdown tool, and due-date reference all in one.
The estimate is especially practical for users searching for a quick calculator to answer questions like how many weeks pregnant am I, what trimester am I in, or when is my due date. It is less useful when cycles are highly irregular or when a clinician has already assigned an ultrasound-based obstetric estimate that differs from menstrual dating.
NHS — Due date calculator — Public-health explanation of how due dates are estimated and why most babies are not born exactly on that date.
NCBI Bookshelf — Pregnancy Dating — Clinical overview of gestational age estimation and the role of ultrasound in setting the best obstetric estimate.
Using the as-of date for appointments and past dates
Many pregnancy week calculators only calculate against today's date, but real users often need a different reference date. You may want to know how far along you were at a booking appointment, how many weeks you will be at a scan, or what gestational age a note from a previous visit implied. The as-of date field lets the tool behave more like a practical gestational age calculator rather than only a live countdown.
The calculation still uses completed calendar days divided into weeks and days. For example, if the LMP-equivalent start date is 1 January and the as-of date is 2 April, the tool reports 13 weeks 0 days. Changing the as-of date updates the weeks pregnant result, trimester, progress percentage, fetal-age approximation, and milestone dates consistently.
The remainder after complete weeks is shown as extra days, which is the usual weeks-plus-days format used in pregnancy dating.
What cycle length changes and what it does not change
When you use LMP mode, the live calculator lets you enter an average cycle length. That setting adjusts the estimated conception date and due date because ovulation usually occurs later in longer cycles and earlier in shorter cycles. It does not rewrite the completed weeks since the first day of the last period, because gestational age is conventionally counted from that LMP anchor.
This distinction matters because some searches mix several intents together: LMP calculator, pregnancy due date calculator, pregnancy weeks calculator, and conception date calculator. A cycle-length adjustment can make the due-date estimate feel more realistic, but a first-trimester ultrasound or clinician-assigned EDD may still be the better dating source when the LMP is uncertain.
Milestone dates shown by the calculator
The result panel turns a raw pregnancy-weeks answer into a planning timeline. It shows the end of the first and second trimesters, the typical 18- to 22-week anatomy-scan window, the 37-week mark, the 39-week mark, and later-term reference points. These dates are not appointment orders, but they make the weeks pregnant result easier to interpret.
This is one way the page intentionally goes beyond a bare how many weeks pregnant am I calculator. The live result gives a current answer, a due-date countdown, fetal-age context, and the practical pregnancy milestones people usually look up next.
Worked example: known due date converted to weeks pregnant
Suppose a scan report lists an estimated due date of 8 October 2026 and you want to know the pregnancy weeks on 2 April 2026. The calculator works backward 280 days from the due date to an LMP-equivalent date of 1 January 2026, then counts forward to the as-of date. That gives 13 weeks 0 days pregnant.
The same calculation estimates conception around 15 January 2026, places the pregnancy in the first trimester under the calculator's trimester boundaries, and shows the 37-week mark on 17 September 2026. In real care, the scan report date should take priority over a fresh LMP estimate if your maternity team has already assigned a due date.
Frequently asked questions
How is gestational age in weeks calculated?
Gestational age is counted from the first day of the last menstrual period (LMP), not from conception. Week 1 begins on the day of the LMP. At the time of a positive pregnancy test, a typical pregnancy is already 4-5 weeks along by this convention.
What trimester am I in?
The first trimester spans weeks 1-13. The second trimester is weeks 14-26. The third trimester is weeks 27 to delivery. Trimester definitions vary slightly between sources; some use 12 weeks as the first/second boundary.
Is gestational age the same as fetal age?
No. Gestational age is counted from the LMP and is approximately two weeks longer than embryonic age (counted from fertilisation). A 12-week gestational age pregnancy has a 10-week-old embryo. Healthcare providers use gestational age consistently to avoid confusion.
Can I calculate pregnancy weeks for a future scan or past appointment?
Yes. Use the as-of date field for the date you want to evaluate. The calculator will show how many completed pregnancy weeks and extra days apply on that date, which is useful for checking how far along you may be at a scan, appointment, travel date, or past note.
Does average cycle length change how many weeks pregnant I am?
The average cycle length changes the estimated conception date and LMP-based due date, but it does not change the number of days counted from the LMP itself. If your cycle is long or irregular and the LMP date does not seem to match an ultrasound, use the clinician-assigned due date or scan estimate for medical planning.
Why does the calculator show fetal age as about two weeks less?
Gestational age starts from the first day of the last period. Fetal or embryonic age is usually approximated from conception, which often occurs around two weeks later in a 28-day cycle. The fetal-age estimate is therefore a rough orientation aid, not a precise fertilisation date.
Which is more accurate: LMP, due date, or ultrasound?
A confident LMP can be useful for an initial estimate, but early ultrasound is often more reliable when cycles are irregular, the LMP is uncertain, or scan measurements differ from calendar dating. If a clinician gives you an estimated due date, use that date for medical decisions rather than trying to override it with an online calculator.
Why do some websites use different trimester boundaries?
Trimester boundaries are not always presented identically. This calculator treats weeks 1-13 as the first trimester, weeks 14-27 as the second trimester, and week 28 onward as the third trimester. If your clinic uses a slightly different convention, follow the clinic's wording when scheduling care.
Can this calculator handle IVF or embryo transfer dating?
No. IVF and embryo-transfer pregnancies use transfer date and embryo age rather than the standard LMP or due-date back-calculation used here. Use an IVF-specific due date calculator or your fertility clinic's instructions for embryo-transfer dating.
What if I am already past my estimated due date?
The calculator can still show completed pregnancy weeks and days past the estimated due date, but post-due-date management is a clinical question. If you are past your due date, follow the monitoring and next-step plan from your maternity team rather than relying on a web estimate alone.
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