Calculate Pregnancy Week By Due Date | Simple Timeline

Finding the right week of pregnancy can feel confusing when every app, friend, and calendar seems to give a slightly different answer. The good news is that there is a clear method behind the numbers your midwife or doctor uses, and you can use the same steps at home once you know your estimated due date on your own.

This guide walks through how due dates are set, how to work out the pregnancy week from a due date in a simple way, and what the week number actually means for you and your baby. You will also see worked examples and a quick reference table so you can double check your own dates.

How Due Dates Set The Pregnancy Week Count

Pregnancy care teams all start with the same basic rule. A full term pregnancy is counted as 40 weeks, or 280 days, from the first day of the last menstrual period, often called the LMP. From that starting point they work forward to find the estimated delivery date, also called the EDD or due date.

Large medical groups describe this clearly. One example comes from Mayo Clinic, which notes that providers count ahead 40 weeks from the start of the last period to estimate the due date and then confirm it with early ultrasound when neededMayo Clinic prenatal care guidance.

Once that due date is set, every pregnancy week is a step along that same 40 week line. Week 1 begins on the first day of the last menstrual period, while conception usually happens about two weeks later. Week 40 ends on the due date.

Pregnancy Week Range	Trimester	Typical Focus At Visits
Weeks 1–4	First	Dating the pregnancy, due date estimate, basic blood tests
Weeks 5–8	First	Heartbeat check, early scan when needed, early symptom support
Weeks 9–13	First	Screening options, early anatomy checks, lifestyle questions
Weeks 14–20	Second	Detailed anatomy scan, measuring growth, feeling first movements
Weeks 21–27	Second	Growth checks, glucose screening, birth plan first talks
Weeks 28–36	Third	More frequent visits, position checks, birth and feeding plans
Weeks 37–40+	Third	Full term monitoring, signs of labour, options if pregnancy goes past due date

Understanding this timeline helps make sense of the calculation. If the due date marks the end of week 40, counting back along this line tells you which week you are in on any given day.

Calculate Pregnancy Week By Due Date For Everyday Planning

To work out the current week from a known due date you can reverse the usual calculation. Instead of starting from the last menstrual period and counting ahead, you start from the due date and count backward.

Step One: Confirm How Your Due Date Was Set

Before using any numbers, check how the due date was chosen. The most common approach uses the first day of the last menstrual period and adds 280 days. When cycles are irregular or dates are uncertain, early ultrasound is used to adjust the estimate. Professional groups such as the American College of Obstetricians and Gynecologists recommend that an early ultrasound be used to confirm or change the date when there is a mismatch between LMP and scan findingsACOG due date methods.

If your due date came from in vitro fertilisation, your team will usually base it on the known conception or embryo transfer date plus a fixed number of days. In every case the result is a single EDD that your team uses for the rest of the pregnancy unless there is a strong reason to adjust it.

Step Two: Count How Many Days You Are From The Due Date

Once the EDD is clear, look at today’s date and count the number of days between today and the due date. If today is before the due date, this number is how many days remain in the standard 280 day window. If today is after the due date, it shows how many days past the original end of week 40 you are.

A calendar, phone app, or simple online day counter can handle the date maths for you. The detail that matters for the week number is the total difference in days.

Step Three: Convert Days To Weeks And Days

Next, subtract the number of days remaining from 280. The result tells you how many days of pregnancy have passed on the standard scale. Divide that number by 7 to find how many full weeks are complete and how many extra days sit on top of that week number.

As one example, if 196 days have passed since the start of the pregnancy count, that equals 28 full weeks, because 28 × 7 equals 196. If 199 days have passed, that equals 28 weeks and 3 days. This is the way providers record pregnancy age in notes and ultrasound reports.

Worked Examples Using A Due Date

Sometimes the steps feel clearer with real dates. These sample timelines show how to work out the pregnancy week from a due date in day to day life.

Example One: Today Is Well Before The Due Date

Say your due date is 10 October. The standard 280 day count ends on that day. If today is 1 August, there are 70 days between 1 August and 10 October. Subtract those 70 days from 280. The result is 210 days.

Divide 210 by 7. The result is 30 full weeks. That means that on 1 August your pregnancy age is 30 weeks and 0 days. On 2 August it will be 30 weeks and 1 day, and so on.

Example Two: Today Is Close To The Due Date

Now take the same due date of 10 October, but today is 5 October. The gap between 5 October and 10 October is 5 days. Subtract 5 from 280 and you have 275 days.

Divide 275 by 7. That gives 39 weeks and 2 days, because 39 × 7 equals 273, with 2 days left over. On 5 October the pregnancy age is 39 weeks and 2 days. By 10 October, the age reaches 40 weeks and 0 days.

Example Three: Pregnancy Continuing Past The Due Date

If the same pregnancy continues beyond 10 October, the day count continues past 280. On 12 October, two days after the due date, the total day count reaches 282. Divide 282 by 7 and you see that this equals 40 weeks and 2 days.

Many babies arrive after the expected date, so this situation is common. Providers often increase monitoring near and past week 40 and talk through options if pregnancy reaches 41 or 42 weeks.

How This Relates To Lmp And Ultrasound Dates

At first the system can feel odd because you may have been given several different numbers during early visits. You might have heard a week count based on LMP and then a slightly different week count after the first ultrasound scan. Once the dating scan is complete, the agreed due date almost always stays fixed.

Health agencies describe three main tools for dating pregnancy: menstrual history, clinical examination, and ultrasound measurements. Early ultrasound tends to give the most reliable single point estimate, especially when cycles are irregular.

After that point, every method that would calculate pregnancy week by due date should match what your official notes say. If there is ever a mismatch, your midwife or doctor can explain which week count they are using and why.

Sample Due Date And Pregnancy Week Table

The table below shows worked examples that you can use as a pattern when you calculate pregnancy week by due date for your own calendar. The exact dates are samples, but the maths stays the same for any due date and any check date. Simple notes keep dates clear.

Due Date	Check Date	Pregnancy Age
10 October	1 August	30 weeks 0 days
10 October	5 October	39 weeks 2 days
10 October	12 October	40 weeks 2 days
20 March	1 January	28 weeks 5 days
20 March	10 February	33 weeks 3 days
5 June	15 April	32 weeks 2 days
5 June	30 May	39 weeks 3 days

When To Ask For Help With Pregnancy Dating

If you ever feel unsure about your pregnancy week after checking dates on your own, raise the question at your next visit. Let your provider know which due date you were given, how you worked out the current week, and where the numbers seem to differ from theirs.

They can review the original scan reports, period history, and any later test results, then confirm whether the current due date and week count still fit. In some situations they may decide to adjust the due date based on ultrasound measurements, especially when early scans and later growth patterns do not match.

Once that decision is made, write the agreed due date in a place you see often. From that point, every time you work out your pregnancy week from the due date you will land on the same week count that appears in your medical notes, which makes the whole process simpler and less stressful.