How pregnancy weeks are calculated: A simple guide

Pregnancy weeks are calculated from the first day of your last menstrual period, not from the date of conception. This method is used because it gives a consistent starting point and helps estimate the due date more accurately. This article explains how this calculation works and why it matters for both mother and baby.

You might assume that pregnancy is a straightforward nine month timeline from the moment you conceive to the day you give birth, but that is not always correct. Many women are surprised to learn that pregnancy weeks usually begin with the first day of their last menstrual period, known as the LMP. This can feel confusing because it covers a time when they are not yet pregnant¹. However, since most women only realise, they are pregnant a few weeks after missing a period, starting the count from the LMP often provides a clearer and more reliable estimate of the due date.

The reason pregnancy is measured in weeks, not months

According to the NHS the average length of a full-term pregnancy is 280 days or 40 days.²The reason it is counted in weeks and days, not months, is because it is a more precise way of ascertaining where a woman is in her pregnancy rather than calculating it in calendar months, which have varying lengths (i.e.. between 28 and 31 days so potentially more, or less, than four weeks). Pinpointing more precisely how far along a woman is in her pregnancy helps to track both the mother’s pregnancy symptoms and the baby’s development more accurately. It also provides the GP and/or midwife with essential information for monitoring you and your baby and affecting your care.

So, what is the first day of pregnancy?

The first day of pregnancy is usually counted as the date of the first day of your last menstrual period (LMP).

How do you calculate your due date?

For the last 200 years or so, Naegele’s rule (created by obstetrician Franz Naegele in the nineteenth century) has been used to calculate your due date - a method generally seen to be more reliable than scrabbling about and trying to remember the exact, and often unknown, date of conception. This is done by taking the first day of your last period and adding 40 weeks (or 280 days) to your LMP.³

Why rely on a set of criteria established in the nineteenth century when we live in the 21st century and a world of new technology? Well, most healthcare providers will use Naegele’s rule as a general guideline and then combine it with more precise dating techniques such as using digital calculators and apps plus an ultrasound scan (see below). So, whilst Naegele’s rule is a tried and tested method that is still widely used as a guideline in modern obstetrics it works on the assumption all women have a regular (typically 28) day cycle with ovulation falling in the middle (day 14) and given that an estimated 14%-25% of women don’t⁴ , it is not an exact science. Recent research has also highlighted that a number of other factors can significantly influence the length of a pregnancy including ethnicity, height, irregular menstrual cycle, timing of ovulation and the weight of the mother.⁵

What happens if you don’t know your LMP?

If you generally have an irregular menstrual cycle, or you just can’t remember your LMP, your periods may not be an accurate predictor of your pregnancy due date, and an ultrasound scan can offer more clarification. This usually takes place in the first trimester (typically between weeks 8-13) and determines how far along you are in your pregnancy by measuring your baby’s crown-rump length – the literal length of the foetus - to determine the gestational age. Your GP or midwife may compare your estimated due date based on the approximate time of your last period with any details from the ultrasound. If there is a discrepancy between the LMP date and the ultrasound date, the ultrasound is generally preferred. This gathered information will be used to determine the type of care, monitoring and tests you may need. All women will be offered a first trimester dating scan as part of routine NHS antenatal care.⁶

Conception date vs gestational age: what’s the difference?

Gestational age refers to the age of pregnancy – counted from the first day of your LMP -whilst the conception date is the foetal age (that is, the age of your baby). Gestational age is typically two weeks longer than the conception age because it includes the time from your last monthly period to ovulation, which is when conception occurs. If you have conceived using IVF or other assisted reproduction techniques gestational age is generally calculated from the date of the embryo transfer plus the age of the embryo at the point of transfer. An IVF due date calculator will also likely be used.

How pregnancy weeks are divided

Pregnancy is divided into three main stages, or trimesters, comprising around three months each and these major phases correspond to significant changes in foetal development and the mother’s body. They matter in terms of tracking pregnancy symptoms and the baby’s growth and development. Each trimester has a unique set of milestones and helps parents-to-be and healthcare practitioners to track progress, plan medical visits and build a clearer, more 3D picture (quite literally, in the case of scans) of what to expect during the pregnancy. There is more information on every stage of your baby’s development on the NHS Best Start in Life hub but as a brief guide:

1st trimester (conception to 12 weeks). During this time your body will be adjusting to the early stages of pregnancy, and you may experience morning sickness, extreme tiredness and mood swings. In this trimester your baby grows quicker than at any other time. By week six you can usually hear a heartbeat and by the end of the trimester your baby’s bones, muscles and vital organs have formed.⁷ A first trimester ultrasound taken at this time usually determines the most accurate due date.
2nd trimester (13 weeks to 27 weeks). This is generally considered the most ‘comfortable’ of the trimesters when morning sickness has usually subsided and you have more energy. You will start to look more pregnant with a growing baby ‘bump’ and you will usually start to feel your baby moving from around week 17.⁸ You might have more trouble sleeping, develop constipation and/or piles; have leg cramps and may experience unusual food cravings. Between 18-21 weeks you may be offered a 20 week screening scan to look for 11 different conditions in your baby, including Down’s and to check your baby’s growth.⁹ If you have a due date ultrasound in this trimester it is suggested that the date could be off by around a week to 10 days.
3rd trimester (28 weeks until birth). You may start to experience new symptoms such as nosebleeds, indigestion or a heightened sense of smell.¹⁰ Your baby will continue to grow rapidly. During this trimester you should have regular antenatal checks, especially around weeks 36 and 40, where your midwife will monitor your blood pressure, urine and fundal height (the size of your ‘bump’) and generally keep an eye on you and your baby’s health. At week 34 your midwife will give you information about preparing for labour and birth and your birth plan will be discussed. At week 38, as your due date approaches your midwife will talk to you about what happens if your pregnancy lasts longer than 41 weeks.¹¹

Why is it important to know how pregnancy weeks are calculated?

The reason it is helpful to understand how pregnancy weeks are calculated is because it is key to planning tests and scans, monitoring the baby’s growth and development according to the three trimesters. This can be useful to spot any potential problems as early as possible, helping your GP or midwife provide the most appropriate prenatal care and also potentially helping the mum to be to be more mentally prepared. In other words, it is important so that you and your baby receive the right medical care at the right time. It also helps your doctor and/or midwife to understand if your baby was born prematurely or past your due date, which are essential for delivery and postpartum care.

FAQs

‘My GP says I’m 6 weeks pregnant, yet I only conceived 4 weeks ago, how can that be right?’
Conception typically happens around two weeks after your last period begins. But to establish your estimated due date your GP or midwife counts 40 weeks ahead from the start of your last period. Why this can be confusing, is that this means that your period is counted as part of your pregnancy timeline, even though you weren’t actually pregnant at that time.

I have been given a couple of estimated due dates, which one should I trust?
Initially, when you first discover you are pregnant you may have been given a couple of estimated due dates as your GP or midwife tries to establish the most accurate one. It might sound obvious but make sure that both you and your healthcare provider are using the same one. A due date is still an estimate, however, and most babies are born between 38 and 41 weeks of pregnancy. In fact, one large study has revealed that just 5% of women actually give birth on their due dates¹² – so try to think of yours as more as a guide to understanding how far along you are in your pregnancy rather than a cast-iron guarantee of when your baby will be born.

How do you track pregnancy weeks when using IVF or other assisted conception techniques?
Since your exact transfer date is known, an IVF due date is generally more precise than an estimated due date in a natural conception – making tracking your pregnancy week by week according to the three trimesters potentially easier. This becomes particularly important as IVF pregnancies are known to have a slighter higher risk of complications.¹³ Keep a close eye on what’s happening and let your GP or midwife know if you have any concerns.

What happens if your due date changes?
There are a range of reasons why your due date could potentially change or was miscalculated. Having irregular periods and getting an ultrasound in the second trimester can both make a due date less precise. But a change to your due date shouldn’t affect your prenatal care too much. Keep up to date with every appointment and scan and if you have any concerns talk them over with your GP and/or midwife. Be mindful that due dates are not an exact science and there’s no real way of knowing the precise day your baby will be born – which should only add to the surprise and unpredictability of parenting.

Why if pregnancy is 40 weeks, do we usually refer to it as a 9 month experience?
Pregnancy is generally referred to as lasting nine months because the standard calculation is based on a 40 week (280 day) cycle and so 9 months is an approximation or simplification given that most people think of months as being four weeks long. In actual fact, a full-term pregnancy can be closer to 10 months (or 41 weeks) as calendar months are not consistently four weeks and can vary between 28 and 31 days.

*If you are at all worried or confused about how pregnancy weeks or your due date are calculated always talk to your GP or midwife.