Pregnancy Week Calculator: How Many Weeks Pregnant Am I?

A pregnancy week calculator estimates how many weeks pregnant you are using the first day of your last menstrual period (LMP). This is the standard method used throughout the UK to date a pregnancy, monitor your baby's development and estimate your expected due date (EDD).

This guide explains how pregnancy weeks are calculated, why dating begins before conception, how ultrasound scans may change your pregnancy week, and why your baby's actual age is different from your gestational age.

How Are Pregnancy Weeks Calculated?

Pregnancy weeks are usually calculated from the first day of your last menstrual period rather than the day conception occurred.

Although fertilisation typically happens around two weeks later, the exact date is rarely known. Using the first day of your last menstrual period provides a consistent way for healthcare professionals to date every pregnancy.

Your pregnancy week is used to plan antenatal appointments, screening tests, ultrasound scans and to monitor your baby's growth throughout pregnancy.

Starting Point	Calculation
First day of your last menstrual period (LMP)	Pregnancy dating begins here.
Average pregnancy length	40 weeks (280 days)
Standard method	Last menstrual period (LMP)

Why Does Pregnancy Start Before Conception?

Pregnancy is dated from your last menstrual period because it provides the most reliable reference point.

Most women know when their last period started, but very few know the exact moment fertilisation occurred. Dating pregnancy from the last menstrual period creates one standard timeline for every pregnancy.

This approach allows GPs, midwives and obstetric teams to assess fetal growth, schedule routine scans and compare pregnancy milestones consistently.

Pregnancy Week	What Usually Happens
Week 1	Your menstrual cycle begins.
Week 2	Your body prepares for ovulation.
Week 3	Fertilisation may occur.
Week 4	Implantation may begin.

Is Your Pregnancy Week the Same as Your Baby's Age?

No. Your pregnancy week and your baby's actual developmental age are not the same.

Pregnancy weeks are counted from your last menstrual period, while your baby's development begins after fertilisation. As a result, your baby's developmental age is usually around two weeks younger than your pregnancy week.

For example, if you are 10 weeks pregnant, your baby has typically been developing for around 8 weeks. This difference is completely normal and is part of the standard pregnancy dating system used worldwide.

Pregnancy Week	Baby's Approximate Developmental Age
6 weeks	About 4 weeks
10 weeks	About 8 weeks
20 weeks	About 18 weeks
30 weeks	About 28 weeks

Can an Ultrasound Change Your Pregnancy Week?

Yes. An early pregnancy ultrasound may provide a more accurate estimate of your gestational age than your last menstrual period.

If you have irregular menstrual cycles, are unsure when your last period started, or ovulated earlier or later than expected, the pregnancy week calculated from your last menstrual period may not reflect your baby's actual development.

During the first trimester, a dating scan measures your baby's crown-rump length (CRL). This measurement is one of the most reliable ways to estimate gestational age and may be used to adjust your expected due date (EDD) if necessary.

Situation	How Pregnancy Is Usually Dated
Regular menstrual cycle	Last menstrual period (LMP)
Irregular menstrual cycle	Dating ultrasound scan
Unknown LMP	Dating ultrasound scan
IVF pregnancy	Embryo transfer date

How Are Pregnancy Trimesters Calculated?

Pregnancy is divided into three trimesters, each representing a different stage of your baby's development.

Knowing your pregnancy week also tells you which trimester you are in. Healthcare professionals use trimesters to plan routine screening tests, monitor fetal growth and discuss changes that typically occur during each stage of pregnancy.

Trimester	Pregnancy Weeks	Main Focus
First trimester	Weeks 1–13	Early development and organ formation
Second trimester	Weeks 14–27	Rapid growth and fetal movement
Third trimester	Weeks 28–40	Growth, maturation and preparation for birth

Each trimester includes important antenatal appointments and screening tests, making accurate pregnancy dating an essential part of your maternity care.

How Accurate Is Pregnancy Week Calculation?

For most pregnancies, calculating pregnancy weeks from your last menstrual period provides a reliable estimate.

However, no dating method is perfect. Factors such as irregular menstrual cycles, variations in ovulation and uncertainty about your last menstrual period can all affect the estimated pregnancy week.

If necessary, your healthcare professional may use an early ultrasound scan to confirm or adjust your gestational age. In many cases, the dating scan performed during the first trimester provides the most accurate estimate.

Good to know: Pregnancy week calculation is designed to monitor your baby's development and guide maternity care. It should be viewed as a medical estimate rather than an exact measurement.

What Can Affect Your Pregnancy Week Calculation?

Several factors can influence how your pregnancy is dated.

Most pregnancies follow the standard dating method based on the last menstrual period, but some situations require a different approach.

Irregular menstrual cycles
Early or late ovulation
An uncertain last menstrual period
Dating ultrasound findings
IVF treatment
Multiple pregnancy (twins or more)
Certain maternal or fetal medical conditions

These factors do not necessarily indicate a problem with your pregnancy. They simply help your healthcare team determine the most accurate gestational age.

When Should You Contact Your GP or Midwife?

If you think you may be pregnant, arrange an appointment with your GP or contact your local maternity service as soon as possible.

You should also seek medical advice if:

You are unsure of your last menstrual period.
Your menstrual cycles are irregular.
You conceived through IVF or fertility treatment.
You experience heavy bleeding or severe abdominal pain.
You have a long-term medical condition that may affect your pregnancy.

Early antenatal care helps confirm your pregnancy, estimate your gestational age accurately and ensure that you receive the appropriate care throughout your pregnancy.

Frequently Asked Questions

How can I calculate how many weeks pregnant I am?

Pregnancy is usually calculated from the first day of your last menstrual period (LMP). Your healthcare professional may use an early ultrasound scan to confirm or adjust your gestational age if necessary.

Why is my pregnancy dated from my last period instead of conception?

The exact date of fertilisation is rarely known, whereas the first day of your last menstrual period is usually easier to identify. This provides one consistent method for dating pregnancies.

Can my pregnancy week change after an ultrasound?

Yes. If an early dating scan suggests your baby is developing differently from the date estimated using your last menstrual period, your pregnancy week and estimated due date may be revised.

Is gestational age the same as my baby's actual age?

No. Gestational age is measured from your last menstrual period, while your baby's development begins after fertilisation. Your baby's developmental age is therefore usually around two weeks younger.

Can I calculate my pregnancy week if I have irregular periods?

Yes, although the estimate may be less accurate. If your menstrual cycles are irregular, an early dating scan usually provides the most reliable assessment of gestational age.

How are pregnancy weeks calculated after IVF?

For IVF pregnancies, gestational age is calculated using the embryo transfer date and the age of the embryo, rather than the last menstrual period.

Why is knowing my pregnancy week important?

Your pregnancy week helps determine when screening tests, ultrasound scans and routine antenatal appointments should take place. It also allows your healthcare team to monitor your baby's growth and development throughout pregnancy.

Will my pregnancy always last exactly 40 weeks?

No. A full-term pregnancy can naturally end anytime between 37 and 42 weeks. The estimated due date is a guide rather than the exact day your baby will be born.

Key Takeaways

Pregnancy weeks are usually calculated from the first day of your last menstrual period (LMP).
Gestational age and your baby's developmental age are not the same.
An early dating scan may provide a more accurate estimate of your pregnancy week.
Knowing your pregnancy week helps plan antenatal care and monitor your baby's development.
Most pregnancies are dated consistently using the same internationally recognised method.

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Medical References

National Health Service (NHS). Your Pregnancy Week by Week.
National Institute for Health and Care Excellence (NICE). Antenatal Care.
Royal College of Obstetricians and Gynaecologists (RCOG). Pregnancy Information.
American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date.
World Health Organization (WHO). WHO Recommendations on Antenatal Care.