Find Out How Many Weeks Pregnant You Are | Clear Methods

That faint line on a test changes everything, and the next question is usually the same: how far along am I? Knowing your pregnancy week helps you plan appointments, read week-by-week guides, and spot when something feels off. The tricky part is that dating a pregnancy does not start when you think it does, and different methods can give slightly different answers.

This guide walks through the main ways to check how many weeks pregnant you are, how doctors and midwives use each method, and what to do when your cycle is irregular or you conceived with treatment. By the end, you will know which date to write in your planner, which number to tell friends and family, and when it makes sense to ask your care team to review it.

How Pregnancy Weeks Are Counted

Pregnancy weeks are counted from the first day of your last menstrual period, not from the day you had sex or the day of ovulation. This is called the LMP method. It might feel strange, because you were not pregnant during the first part of that cycle, yet that time still counts in the total.

If you have a regular 28-day cycle, ovulation usually happens around day 14. By the time a fertilized egg implants, you are already about four weeks pregnant by this counting method. Health services and hospital systems use this approach because most people can remember the first day of a recent period more easily than a possible conception date. Many resources also describe pregnancy as a 40-week span based on this method, with full term often placed between 37 and 42 weeks.

Because this system starts earlier than conception, it helps to think of your pregnancy week as a shared language. Your care team, scan reports, and pregnancy apps all use that same clock so everyone talks about the same point in time.

Find Out How Many Weeks Pregnant You Are With Your Last Period

The simplest way to estimate your pregnancy week at home uses your last menstrual period. You only need a calendar and the date when your most recent normal period began.

Step-By-Step LMP Method

Use this quick process:

• Write down the first day of your last normal period.
• Count forward in weeks from that date to today. Each seven-day block is one full week.
• If today lands partway through a week, add those extra days to the week number.
• Share that result at your first prenatal visit so your provider can compare it with scan findings.

Many people like to count by months, but months vary in length. Weeks give a clearer picture for scan timing, screening tests, and growth checks. That is why most medical charts and pregnancy guides use week numbers instead of months.

What If Your Cycle Is Not 28 Days?

Standard calculators often assume a 28-day cycle. If your cycles are shorter, you may ovulate earlier; if they are longer, you may ovulate later. In that case, your LMP-based week count can be off by several days or even more than a week.

If you track ovulation with test strips, basal temperature, or cervical mucus, you might already know when ovulation tends to happen. You can then adjust your estimate slightly. Still, most providers rely on early ultrasound to fine-tune the date when cycles are irregular or periods are hard to recall.

Using Ultrasound To Date Pregnancy

Ultrasound plays a big role in working out how many weeks pregnant you are. In early pregnancy, measurements of the embryo or fetus are very consistent from one pregnancy to the next, so size lines up closely with age in weeks.

An early scan, often called a dating scan, usually takes place between about 8 and 14 weeks. During this scan, the sonographer measures the length of the embryo or fetus from head to bottom, known as the crown-rump length. That number feeds into formulas that estimate how many weeks have passed since your LMP-based start date.

If the scan age and your LMP estimate match within a few days, your care team may keep the LMP date. If the difference is larger, and the scan is early enough, the scan date often becomes the lead date for your due date and week count. Later scans still help check growth, but they are less accurate for setting the starting point.

Main Ways To Work Out Your Pregnancy Week

The table below gathers the main tools you are likely to meet and shows when each type works best.

Method	What It Uses	Best Time To Use
Last Menstrual Period (LMP)	First day of your last normal period and calendar counting	From the moment you miss a period and take a test
Early Ultrasound Dating Scan	Crown-rump length or early fetal measurements	Roughly 8–14 weeks of pregnancy
Later Ultrasound Checks	Head, abdomen, and femur measurements	After the first trimester to track growth; rarely used to reset dates
Online Due Date Calculator	LMP, sometimes cycle length or conception date	Any time at home as a quick check
IVF Or Fertility Treatment Dates	Egg retrieval date, embryo age, or transfer date	From treatment onward, often paired with early scans
Ovulation Tracking Records	Ovulation test strips, basal temperature charts	When cycles are irregular and LMP feels unreliable
Physical Exam	Uterus size and fundal height measurement	More helpful after the first trimester as a cross-check

Online Calculators And Apps To Estimate Pregnancy Weeks

Many people use online pregnancy tools on the same day they see a positive test. These calculators ask for your LMP, and sometimes your average cycle length, and then give an estimated due date and current week count.

The NHS pregnancy due date calculator lets you plug in the first day of your last period and see an estimated due date along with a basic week count. A similar approach appears in the Cleveland Clinic due date calculator, which explains that due dates are usually set 40 weeks from that LMP date.

You can also read through the March of Dimes guide on calculating your due date, which walks through how due dates are estimated and why babies often arrive on slightly different days. For an overview of how pregnancy unfolds week by week, the Mayo Clinic pregnancy week-by-week overview helps you line up your current week with common milestones.

These tools are handy, yet they are still estimates. Early scans, blood tests, and your medical history give your care team more detail. Treat every calculator result as a starting point that you can bring into a visit, not as a fixed rule.

Pregnancy Weeks, Trimesters, And Months

Once you know how many weeks pregnant you are, it helps to translate that number into trimesters and rough months. Pregnancy is often described in three parts: early, middle, and late. Each part spans a range of weeks, and there is some variation between sources, but the pattern stays similar.

Many guides divide the first trimester from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until birth around week 40. A full-term baby may arrive a bit earlier or later than that 40-week mark, so your exact delivery date can still surprise you even with a well-dated pregnancy.

Trimester And Week Overview

This table links week numbers, trimesters, and a few things many people notice during each stretch.

Trimester	Weeks	Common Features
First Trimester	Weeks 1–13	Tiredness, nausea, breast tenderness, first scan and heartbeat checks
Second Trimester	Weeks 14–27	Growing bump, first movements, anatomy scan, more steady energy for many
Third Trimester	Weeks 28–40+	More frequent visits, stronger movements, birth planning, monitoring for complications

Common Situations That Change Your Pregnancy Week Count

Not every pregnancy fits the standard cycle pattern. Some people have irregular periods, some use fertility treatment, and some carry twins or more. In each of these situations, your care team may lean more on scans and specific dates from treatment records than on the LMP alone.

Irregular Cycles Or Uncertain Last Period

If your periods are widely spaced, unpredictable, or affected by conditions such as polycystic ovary syndrome, the LMP start date may not match your ovulation timing. You may not remember the last full bleed, or you may have spotting that makes the starting point hard to pick.

In these cases, early ultrasound gains weight. A scan in the late first trimester can give a week count that lines up closely with average growth patterns. Your provider may base your due date on that scan, then use fundal height and later scans to confirm that growth stays on track with that date.

IVF Or Other Fertility Treatment

If you conceived with in vitro fertilization or another treatment, you likely have precise records of egg retrieval, embryo growth in the lab, and the transfer date. Clinics use these details to assign a pregnancy week even before a scan.

For an IVF cycle, the due date often starts from the embryo age at transfer plus the standard 280-day span used for other pregnancies. An early dating scan still plays a role, but the treatment timeline carries a lot of weight, so your LMP date may be less relevant.

Twins Or More

For twins or higher-order multiples, the due date itself is usually set with the same methods: LMP, IVF dates, and early ultrasound. The key difference is that many twin pregnancies are born before 40 weeks. Your care team may plan closer monitoring, extra scans, and different timing for birth.

Because growth patterns can shift in multiple pregnancies, your week count may stay the same but your care plan may change. That is why clear dating early on helps later decisions about monitoring and timing.

How Often Providers Check Your Pregnancy Week

Once you have a working due date, each prenatal visit links back to that number. At early appointments, staff may reconfirm your LMP, ask about cycle length, and schedule a dating scan if you have not had one yet. They may also order blood tests that line up with specific week windows.

During the second trimester, your week count helps schedule screening tests and the detailed anatomy scan. As weeks pass, your midwife or doctor measures fundal height from the pubic bone to the top of the uterus. That number in centimeters often matches your week count and works as a rough check that growth keeps pace.

Near the end of pregnancy, visits usually become more frequent. Your week number guides decisions about induction, monitoring for high blood pressure, and planning for birth after 41 or 42 weeks if you and your baby stay well.

When To Ask For A Review Of Your Dates

Most pregnancies move along without any need to change the original due date. Still, there are times when asking for another look at your dates makes sense.

• Your early scan age and LMP age differ by more than a week, and no one has explained which one your clinic uses.
• Your fundal height measurements fall behind or jump ahead of the week count for more than one visit in a row.
• You have spotting, bleeding, or strong pain, and your symptoms do not match the week you have been given.
• You switched clinics or moved countries, and records from your first scan are missing.

In these situations, ask your provider which date they rely on, what evidence they used, and whether another scan or review would help. Clear dating does not remove every surprise, yet it makes decisions about tests and birth timing easier to plan and understand.

Practical Checklist For Tracking How Many Weeks Pregnant You Are

To keep your pregnancy week straight without stressing over every number, use a simple checklist that you can bring to visits.

Simple Pregnancy Week Checklist

• Write your LMP date on the first page of your pregnancy notebook or app.
• Note your estimated due date from an online calculator and label it as a first guess.
• Add the date and result of your earliest ultrasound, including the week and day written on the report.
• Circle the date your provider finally confirmed as your official due date.
• Update your current week number once a week on the same day, such as every Sunday night.
• Mark key scan and test weeks (such as 12, 20, and the weeks suggested in your country) so you can see what is coming.
• Bring your notes to every visit and ask staff to correct any parts that do not match their records.

With a clear due date, a basic sense of how pregnancy weeks are counted, and a few reliable tools, you can stay on top of your pregnancy timeline without getting lost in dates. Always share any worries about timing, bleeding, or pain with your care team, and use online calculators and apps as guides rather than rulers.