Trimester Calculator: Key Pregnancy Dates by Trimester

How to Use Trimester Calculator: Key Pregnancy Dates by Trimester

1. Step 1: Choose a dating method

   Pick LMP (last menstrual period), Due Date, Conception date, or Ultrasound.

2. Step 2: Enter the required date(s)

   Provide the date for your chosen method. For Ultrasound, add the measured weeks and days.

3. Step 3: Set the “as of” date

   Leave today’s date or pick another date to see trimester status on that day.

4. Step 4: Review trimester dates

   See your current trimester, the start and end dates for each trimester, and your EDD.

5. Step 5: Copy your summary

   Use the Copy button to save a simple summary for notes or sharing.

Key Features

• Trimester start and end dates
• Estimated due date (EDD)
• Flexible methods: LMP, EDD, conception, ultrasound
• Key milestones timeline
• Copyable summary

Understanding Results

Formula

Gestational age (GA) counts from the first day of the last menstrual period (LMP). By convention, a full-term pregnancy is 40 weeks (280 days) from LMP or 38 weeks (266 days) from conception. When you enter an EDD, the calculator derives an equivalent LMP by subtracting 280 days. With ultrasound, it anchors the timeline using the scan date and the measured GA. Conception entries add 14 days to align with gestational age conventions.

Reference Ranges & Interpretation

We use straightforward trimester boundaries: First (0–12w6d), Second (13w0d–27w6d), Third (28w0d to birth). Milestones such as the 20-week anatomy scan and the 37-week term window are included as planning cues. These dates are commonly used in education and care pathways, though specific schedules may vary by clinician and context.

Assumptions & Limitations

Results are educational and cannot replace medical advice. Early ultrasound dating is often the most precise. Cycle variability, late ovulation, IVF protocols, multiple gestations, and individual differences affect actual timing. Always follow your clinician’s guidance if it differs from the general template.

Complete Guide: Trimester Calculator: Key Pregnancy Dates by Trimester

Use the trimester calculator to find your current trimester, start and end dates for each trimester, and due date. Choose LMP, EDD, or ultrasound dating.

What this trimester calculator shows

This trimester calculator is designed to make timing simple. In a few taps, you can confirm which trimester you are in today, see the exact start and end dates for the first, second, and third trimesters, and view your estimated due date (EDD). The tool also highlights the next major milestone so planning appointments and life events feels more manageable. Everything runs in your browser for privacy—nothing is stored or sent anywhere.

You can start from several common dating methods: the first day of your last menstrual period (LMP), a provided due date, a known conception date, or an ultrasound report that lists a measured gestational age. No matter which method you choose, the calculator converts it into the same standard pregnancy timeline used by clinicians. The goal is clear, consistent dates you can use with confidence.

Because the interface is mobile‑first, the inputs and results fit small screens without side‑scrolling or tiny touch targets. Labels are unambiguous, and the outputs use plain language (for example, “First trimester,” “Second trimester,” “Third trimester”) along with clear calendar dates you can mark on your phone or paper calendar. Whether you are planning time off, arranging travel, or simply staying organized, this layout is intended to reduce friction and help you make decisions quickly.

How trimesters are defined

Pregnancy is typically broken into three trimesters. While definitions can vary slightly, a common convention is: First trimester from week 0 through 12 weeks and 6 days, Second trimester from 13 weeks 0 days through 27 weeks 6 days, and Third trimester from 28 weeks 0 days to birth. These boundaries help group care milestones and discussions, from early symptoms and screening decisions to growth assessments and labor preparation.

By convention, gestational age counts from the first day of the last menstrual period (LMP), not from conception. A full-term pregnancy is often described as 40 weeks (280 days) from the LMP. Many clinical conversations and visit schedules are organized around this timeline, which is why tools like a trimester calculator can save time and reduce confusion when you’re looking ahead.

Different organizations and practices may use slightly different labels at the edges, but the boundaries above (0–12w6d, 13–27w6d, 28w0d to birth) are widely recognized and easy to follow. Our calculator uses this standard so your results line up with many educational materials and appointment plans.

It also helps to understand how week labels are used in practice. When we say someone is “in week 18,” it usually means they have completed 17 full weeks and are partway through the 18th week. Our tool displays both the completed weeks + days and a simple week label so you can match how clinicians and educational guides often speak about timing.

Dating methods and conversion

LMP (last menstrual period): If you know the first day of your last period, this is the simplest starting point. The calculator treats that date as day 0. Gestational age is the number of days since the LMP. The estimated due date is LMP + 280 days.

EDD (estimated due date): If your clinician gave you a due date, enter it directly. The calculator reverses the math (EDD − 280 days) to estimate the corresponding LMP anchor and builds the full trimester timeline from there.

Conception date: When you know ovulation, conception, or an embryo transfer date, we convert it to gestational age by adding 14 days. From that anchor, the calculator derives the same trimester dates and EDD (conception + 266 days).

Ultrasound: If you have an ultrasound report that lists gestational age on a scan date (for example, “8 weeks 3 days”), enter the scan date and the measured weeks and days. The calculator treats this pair as a precise anchor and projects forward and backward to your trimester boundaries and EDD.

Regardless of which method you choose, the output is harmonized on one scale so your trimester, week label, and due date are consistent. If dates from different methods disagree, clinicians often prioritize early ultrasound dating, especially in the first trimester, because it tends to be the most precise.

If you are switching methods (for example, moving from LMP to ultrasound once you receive a scan report), you can re‑enter your data using the new method to see how the EDD or trimester boundaries shift. This makes it simple to mirror your clinician’s official dating approach without doing the calendar math by hand.

How we calculate trimester dates

The math is transparent. Once we have a unified timeline anchored to an LMP-equivalent date, the boundaries are straightforward: the First trimester starts on the anchor date and ends at 12 weeks and 6 days; the Second trimester starts at 13 weeks 0 days and ends at 27 weeks 6 days; the Third trimester begins at 28 weeks 0 days and continues to birth. The tool also shows helpful milestones like 20 weeks (common timing for the anatomy scan), 28 weeks (start of the third trimester), 37 weeks (term window begins), and 40 weeks (estimated due date).

On mobile, results are compact and easy to scan. You will see which trimester you are in today, the exact start and end dates for each trimester, your gestational age as weeks and days, and the next milestone with its calendar date. A copy button lets you paste a simple summary into notes or a message so loved ones can follow along.

Some people prefer to print or save a quick snapshot. The copyable summary captures your current gestational age, trimester, estimated due date, and the three trimester ranges in a short text block. Paste it into your notes app, email it to a partner, or bring it to your next appointment as a reference point. You can regenerate the summary anytime as your “as of” date changes.

Worked examples

Example A (LMP-based): Suppose your LMP was March 1. Counting 280 days from March 1 gives an estimated due date in late November. The First trimester runs from March 1 to approximately May 31 (12w6d), the Second trimester runs from June 1 (13w0d) to late August (27w6d), and the Third trimester begins around late August (28w0d) and runs to the due date. As you change the “as of” date, the tool updates which trimester you are in today.

Example B (EDD-based): If your clinician provided an EDD of October 10, the calculator subtracts 280 days to find the LMP anchor (around January 3). From there, the First trimester is approximately January 3 to April 2 (12w6d), the Second trimester is April 3 (13w0d) to July 8 (27w6d), and the Third trimester is July 9 (28w0d) to October 10 (40w0d). All of this appears under “Trimester dates.”

Example C (Ultrasound-based): Your scan on May 8 shows 8 weeks 3 days. The calculator treats that measurement as an anchor. It computes the implied LMP date, lays out trimester boundaries, and estimates your due date automatically. If you revisit later, just set “as of” to today to keep your trimester and week label in sync.

You can build more scenarios to explore “what‑ifs.” For instance, if you only know the conception window within a few days, try entering the earliest and latest plausible dates and compare the outputs. This gives you a reasonable band for the EDD and trimester transitions until a clinician provides an official date. Similarly, if a clinician adjusts your EDD after an early scan, re‑enter that EDD to align your personal calendar with the clinical plan.

Irregular cycles and IVF

Real life rarely fits a perfect 28-day cycle. If your periods are irregular, LMP-based dating may be less exact because ovulation could occur earlier or later than the typical two-week assumption. In those cases, an early ultrasound can be very helpful, and many clinicians rely on it when there’s a discrepancy. If you have a clearly documented conception date (for example, an IVF transfer), using the Conception method can also provide a consistent and transparent timeline.

If you’re planning a pregnancy and prefer estimating ovulation first, our tools can help with broader timing: try the Ovulation Calculator and the Fertile Window Calculator. After pregnancy is confirmed, you can switch to the Pregnancy Due Date Calculator or the Gestational Age Calculator to stay aligned with the clinical timeline.

For IVF, the embryo transfer date offers a clear reference. Many clinics will provide a corresponding gestational age or EDD. If you have either, you can use the EDD method directly or convert your transfer date by adding the appropriate number of days for the embryo’s age at transfer before applying the standard 266‑day conception‑to‑birth interval. When in doubt, enter the EDD your clinic provided—it will always yield a consistent trimester timeline in this tool.

Ultrasound vs. LMP

Early ultrasound measurements (often in the first trimester) are generally the most precise method for dating a pregnancy. If an early ultrasound disagrees with LMP by more than a small margin, clinicians often adjust the due date to match the ultrasound. Later in pregnancy, variability in growth and measurement can widen the error bars, which is one reason early data points are so valuable.

Our calculator allows you to enter the ultrasound scan date and the measured gestational age (weeks + days). It then projects to your trimester dates and due date. If you have both LMP and ultrasound data, try both and compare—this is a quick way to understand why your clinician might favor one method for official dating.

If your scan was performed later in pregnancy, bear in mind that measurement variability increases. In those situations a clinician may rely more on earlier dates or maintain the originally established EDD unless a difference crosses a clinically meaningful threshold. The purpose of this tool is to reflect the logic behind those decisions and keep your personal timeline in step.

What affects accuracy

Several factors influence how closely calendar predictions will match real life: cycle variability, timing of ovulation, first-trimester ultrasound availability, multiple gestations, and individual differences in growth and labor timing. The trimester boundaries are helpful for planning and education, but they are not medical advice and cannot predict exactly when labor will start.

For educational context, many guides consider 37 weeks as the start of the term window, with “early term” typically defined as 37w0d–38w6d. Schedules for visits and tests can vary. Always follow your clinician’s specific plan if it differs from a general template. Authoritative sources such as the American College of Obstetricians and Gynecologists (ACOG) publish patient education on timing and visits; see ACOG for more.

Finally, remember that due dates are estimates, not deadlines. Many births happen before or after the 40‑week mark and still fall within the range of normal. Think of the EDD as the midpoint of a window rather than a precise appointment. The trimester dates and milestones are planning tools to help you prepare—not promises about how your body will behave on a particular day.

Using your results

Once you know your trimester and key dates, you can plan the practical steps that matter: booking appointments, preparing questions, and arranging time away from work or school if needed. Many people also like to set reminders around the anatomy scan window (roughly 20 weeks) and the start of the third trimester (around 28 weeks) for vaccinations, classes, or birth planning conversations.

If you want a closer look at weekly progress, use our Pregnancy Week Calculator to see your current week label and a short week-by-week view. If you only know the due date, the Pregnancy Due Date Calculator produces the same EDD-based timeline. If you want to cross-check how far along you are from a different angle, the Gestational Age Calculator provides the completed weeks and days from any anchor date. For planning weight goals during pregnancy, the Pregnancy Weight Gain Calculator offers guideline-based ranges you can discuss with your clinician.

If you’re curious about timing earlier in the journey, the Conception Date Calculator estimates when conception likely occurred based on your available information. Connecting these tools can give you a fuller picture while keeping the core dates straight.

Consider using your results to prepare a one‑page plan: your current week and trimester, the upcoming milestone date, questions for the next visit, and any logistics (childcare, work coverage, transport). Small, concrete steps reduce stress and create room for the moments you want to enjoy. You can regenerate or adjust this plan as the timeline evolves.

Related tools and next steps

Here are helpful links to continue exploring, each with trailing slashes for quick navigation:

• Pregnancy Week Calculator
• Pregnancy Due Date Calculator (EDD)
• Gestational Age Calculator
• Conception Date Calculator
• Pregnancy Weight Gain Calculator

These tools work together. Use the trimester dates for planning, then zoom in with week-based views as needed. For any concerns or symptoms, contact your clinician—online calculators are educational and cannot replace professional care.

Frequently Asked Questions