How Many Weeks Pregnant Am I? Working Out Your Dates

One of the first questions that comes to mind when you find out you're pregnant is usually: "How many weeks pregnant am I?" Knowing your pregnancy week is the foundation for tracking your baby's development, planning appointments and finding out your due date. This guide explains how the calculation works and what the pregnancy weeks actually mean.

How Is a Pregnancy Week Calculated?

Contrary to what most people assume, pregnancy is counted not from conception but from the first day of your last menstrual period (LMP). The reason is that it's difficult to know exactly which day conception happened. Ovulation and fertilisation generally occur around two weeks after the date of your last period.

Under this method:

• A full pregnancy lasts around 40 weeks (280 days) from the date of your last period.
• Counted from conception, that comes to roughly 38 weeks.
• The NHS defines the normal range for birth as 37 to 42 weeks from the first day of the last period.

Estimated Due Date (Naegele's Rule)

To work out your due date roughly, clinical practice uses Naegele's rule: to the first day of your last period, add 1 year, go back 3 months and add 7 days. That corresponds to a pregnancy of about 280 days.

Important: a calculation based on your last period is an estimate. The ultrasound measurement taken in the first trimester (the baby's crown-rump length) is generally more accurate for establishing gestational age. If the ultrasound differs, your doctor will update your due date accordingly. Always consult your doctor for a definitive date.

Pregnancy Trimesters

Pregnancy is divided into three main periods, or trimesters. Under the definition used by the American College of Obstetricians and Gynecologists (ACOG):

• First trimester: from the last period to 13 weeks and 6 days
• Second trimester: from 14 weeks 0 days to 27 weeks 6 days
• Third trimester: from 28 weeks 0 days to 40 weeks 6 days

Each trimester has its own stages of development and its own things to watch for.

First Trimester (Weeks 1-13)

The baby's essential organs begin to take shape in this period, particularly between weeks 5 and 8. The heartbeat generally starts at 5 to 6 weeks and becomes audible on Doppler at 9 to 12 weeks. Nausea, fatigue and emotional swings are common for the mother.

Second Trimester (Weeks 14-27)

Known as the most comfortable period for most mothers-to-be. Nausea generally eases. The baby's sex can usually be determined at the detailed anatomy scan at 18 to 20 weeks. You may start feeling your baby's movements during this period.

Third Trimester (Weeks 28-40)

The baby grows rapidly and prepares for birth. Breathlessness, back pain and frequent urination are common for the mother. Birth preparations and the hospital bag should be finished during this period.

What Do Term and Preterm Mean?

The timing of birth is also expressed in weeks. According to ACOG:

• Early term: 37 weeks 0 days to 38 weeks 6 days
• Full term: 39 weeks 0 days to 40 weeks 6 days
• Late term: 41 weeks 0 days to 41 weeks 6 days
• Post-term: 42 weeks and beyond

The World Health Organization defines births occurring before 37 weeks as preterm. Where there is a risk of preterm birth, following your doctor's guidance closely matters.

Why Tracking Your Pregnancy Weeks Matters

Keeping track of your pregnancy week helps you follow your baby's development and have the right checks at the right time. Your baby's size, the organs developing and the changes in your own body differ every week.

With Baby Wallie you can enter the date of your last period and have your pregnancy week calculated automatically, then track week-by-week development, your own health data (blood pressure, weight, symptoms) and your appointments in one place. For the questions that come up along the way, you can ask the Wallie AI assistant.

Frequently Asked Questions

Will the calculation be accurate if my periods are irregular? For people with irregular cycles, a calculation based on the last period can be misleading. In that case the most reliable method is the ultrasound measurement taken in the first trimester.

How many weeks am I when the pregnancy test turns positive? When a test comes back positive after a missed period, you're usually 4 to 5 weeks along, because the count starts from the date of your last period.

Always consult your doctor for medical decisions about your pregnancy. This content is for general information and does not replace medical advice.

If you'd rather not do the maths by hand, just enter the date of your last period into our pregnancy calculator: it shows your pregnancy week, month and estimated due date instantly, and takes you to the page for the week you're in.