Pregnancy Week & Key Dates
See how far along you are and what’s coming next
Pregnancy weeks, trimesters and key scans FAQ
What does this pregnancy week calculator actually show?
This tool turns one starting date into a gestational age estimate. It shows roughly how many weeks pregnant you might be today, which trimester that sits in, an estimated due date, and a list of key weeks when appointments and scans often cluster. It is designed to give you a calm overview, not to replace clinical dating.
Why does it ask for LMP or conception date?
Many pregnancy charts start counting from the first day of the last menstrual period (LMP), using a 40-week calendar. Others work from a known conception or ovulation date. If you tracked ovulation or know when treatment happened, conception may feel clearer; otherwise, LMP is often easier to remember. The calculator can work from either and then builds a standard week count.
How are the trimesters broken up?
Different sources draw trimester lines slightly differently, but a common pattern is:
- First trimester: up to around 12–13 weeks.
- Second trimester: roughly 14–27 weeks.
- Third trimester: from about 28 weeks onwards.
The calculator uses similar ranges so that your week number maps onto language you’ll hear in many guides and appointments.
Does the week number here match what my midwife or doctor will say?
Often it will be close, especially early on, but ultrasound and clinical judgement can nudge the official dates. If a dating scan suggests a different start point, your care team may update the working due date and week count. When that happens, the medical version is the one to follow for decisions about scans, screening and timing of birth.
What are the “key dates” in the timeline for?
The timeline highlights common milestones such as the end of the first trimester, typical windows for the 12-week dating scan and the 18–21 week anomaly scan, and the start of the third trimester. These are based on widely used public information and are meant to help you plan questions and expectations, not to prescribe exact appointment days.
Can I use this calculator for IVF, twins or other higher-risk pregnancies?
IVF pregnancies, twins or higher multiples, and pregnancies with medical complications often follow more specialised dating rules and monitoring plans. This page does not handle embryo transfer dates or complex scenarios. Treat the output as broad education only and lean on your fertility clinic or obstetric team for precise dates and thresholds.
Should I use this to decide if I need urgent care?
No. If you experience pain, bleeding, fluid loss, concerning symptoms or reduced movements later in pregnancy, seek urgent assessment from a midwife, doctor or emergency service. Calculators cannot safely judge emergencies and should never be used to decide whether to get help.
How to use this pregnancy week and key dates calculator
This page is designed to give you a single, scrollable view of your pregnancy timeline. Instead of juggling dates in your head, you can see roughly how far along you are, when trimesters switch and where common scan windows sit in your calendar.
1. Choose how to anchor the timeline
Start by picking the dating method that matches what you know best. If you remember the first day of your last menstrual period, choose LMP. If you tracked ovulation, insemination or a likely conception day, choose the conception/ovulation option. The calculator then converts that into an internal “LMP-style” start date so your week number lines up with typical charts.
2. Enter your date and let the tool do the counting
Use the date picker to add your key date, then press Show pregnancy week. The tool will estimate gestational age today in weeks and days, assign a trimester, and show an approximate estimated due date plus a window when many babies arrive.
3. Scan the trimester overview and next key point
Near the top of the summary, you’ll see both your week count and which trimester that falls in. You’ll also see a “next key date”—for example the week 12 dating scan window, the 18–21 week anomaly scan slot, or the start of the third trimester—based on your timeline. This can make it easier to plan appointments, travel or work conversations.
4. Scroll through the week markers
Below the headline snapshot, the calculator lists several anchor weeks (such as weeks 12, 20, 28, 32, 36 and 40) with approximate calendar dates and a short note about what often happens then. These notes are intentionally plain-language so you can quickly get a sense of the road ahead and decide what you’d like to ask at your next visit.
5. Save a copy for later
Use the Copy summary button to drop your snapshot into a notes app, pregnancy journal or message thread. The summary includes your input date, method, week and day estimate, trimester, estimated due date and the typical window, so you can refer back to it without running the calculator again.
Remember that no online calculator can replace personalised care. If the dates or language here don’t match what your midwife or doctor has told you, treat this as background reading only and follow the plan you’ve agreed together instead.
How the pregnancy week and key dates math works
The maths behind this page mirrors common clinical shortcuts for dating pregnancy. It uses simple day counts so you can see how professionals arrive at language like “20 weeks” or “third trimester” without needing a specialised wheel.
1. Converting your date into an LMP-style start
If you enter an LMP, the calculator treats that as day 0 and builds a 40-week timeline from there. If you enter a conception date, it assumes conception occurred about two weeks after LMP in a typical 28-day cycle and subtracts 14 days to create an internal LMP-style date. That way your week count follows standard gestational age charts.
2. Counting forwards to find your due date
From that internal start date, the tool adds 280 days (40 weeks) to estimate a due date. It also marks a rough “term window” by highlighting dates around 37 to 42 weeks, the span in which many full-term births happen in public information resources.
3. Counting backwards from today to find your week
To work out your current week, the calculator compares today’s date to the internal start date and divides the days between them into whole weeks plus extra days. If today falls outside a typical 0–42 week range, it flags that instead of claiming a gestational age that would not make clinical sense.
4. Assigning a trimester and key week markers
The trimester label is then chosen based on your week count using common ranges. Finally, the calculator steps along the calendar to attach dates to important anchor weeks, such as around 12 weeks (dating scan window), 20 weeks (anomaly scan window), 28 weeks (third trimester marker) and 40 weeks (nominal due date). The short text blurbs next to each draw on public pregnancy guides.
Because cycle length, ultrasound findings and health history all change what is safest for you, treat these numbers as a helpful outline rather than exact orders. Your maternity team’s version of your dates and key appointments should always be the one that guides your decisions.
References and further reading on pregnancy weeks and key dates
These resources explain how due dates, gestational age and scan timing are usually worked out:
- NHS — Due date calculator — describes using the first day of the last period to estimate due date and notes that pregnancy normally lasts about 37–42 weeks.
- American College of Obstetricians and Gynecologists — Methods for estimating the due date — outlines how menstrual dates and ultrasound together provide the best obstetric estimate of gestational age.
- NHS — Ultrasound scans in pregnancy — explains typical timing for the 12-week dating scan and 18–21 week screening scan and how they fit into routine pregnancy care.
Use these as background reading, then rely on your own maternity team for personalised dating, scan schedules and decisions about birth timing.