Five months pregnant is how many weeks

In this article

• I'm confused by what week it is in my pregnancy
• What week is week one of pregnancy?
• How many weeks are in pregnancy?
• How many weeks are in a trimester?
• Is my baby really the size you say each week?
• How do I switch from getting pregnancy to baby newsletters each week?

Weeks, trimesters, months - calculating pregnancy dates can be confusing. Especially when you're told you're five weeks pregnant but in week six. Both are right though, because weeks pregnant means weeks completed. And by the time you've completed a week you'll be in the next one. Then there's 40 weeks of pregnancy across three trimesters. Here's how your dates work so it's easier to know what trimester or week it is in your pregnancy.

I'm confused by what week it is in my pregnancy

You've had the big fat positive, your dates have been calculated but your doctor has told you that you're four weeks pregnant and in week five. It's not a mistake though. To understand this, it may help to think about age. When a baby reaches a year old, they'll have completed their first year. They'll be one year old but now in their second year of life.

Similarly, when you complete your first week of pregnancy, you are one week pregnant and already in your second week.

Following on from that you when you're:

• two weeks pregnant you're in your third week
• three weeks pregnant you're in your fourth week
• four weeks pregnant you're in your fifth week
• five weeks pregnant you're in your sixth week

And so on. This is why your personalised weekly newsletters from BabyCentre will tell you that you're four weeks pregnant when you're in your fifth week of pregnancy. Sign up for our newsletters here.

What week is week one of pregnancy?

In most cases week one will begin from the first day of your last period. You'll hear doctors and midwives call this your last menstrual period or LMP. They count the first day of your LMP as day one of your pregnancy (NHS 2018a). This is because, even though you won't have ovulated and conceived until a couple of weeks later, they can't know for sure exactly when.

Using the first day of your LMP, you can also use our due date calculator to find out your estimated due date. You may prefer to count your pregnancy from the day you felt like you were ovulating. This will give you an estimated date of conception that will be about two weeks after the first day of your LMP. However, it won't tie in with the date your midwife, doctor or BabyCentre rely on to tell how pregnant you are.

The due date calculation works best if your menstrual cycle is regular and your periods are every 28 days. If you have an irregular menstrual cycle, you may not know the date of your LMP. And if your cycle varies in length, counting from the first day of your LMP may not give you a date you can rely on.

If you know exactly when you conceived because you had fertility treatment then your dates will be based on this.

How many weeks are in pregnancy?

Counting from the first day of your LMP your pregnancy will be calculated on the basis of 40 weeks, although this doesn't mean that's when you'll have your baby. Your actual date of delivery could be a few weeks before or after. Your first ultrasound scan, called your dating scan, may give you a different, more accurate due date for your baby. You should be offered a dating scan when you're between around 10 weeks and 14 weeks pregnant, based on your LMP (NHS 2017).

How many weeks are in a trimester?

There are 13 or 14 weeks in each trimester. Trimester means 'a period of three months'. As most pregnancies are around nine calendar months long, each pregnancy is divided into three trimesters. Trimesters are a useful way to describe the different phases and what is happening in your body. They are broken down into the following weeks:

• Your first trimester lasts from the very beginning of your pregnancy until you're 13 weeks plus six days pregnant. Along with the excitement of discovering that you're expecting, you may feel exhausted and queasy.
• Your second trimester lasts from when you're 14 weeks to 27 weeks plus six days pregnant. During this trimester you'll probably feel much better. You may start to bloom, your bump will show and early pregnancy symptoms will be behind you.
• Your third trimester lasts from when you're 28 weeks pregnant until you have your baby. You'll need extra energy to help your baby's growth spurt and tiredness will probably return.

You may also hear people talk about the fourth trimester. This refers to the three months after the birth, when you're bonding with your new baby.

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Is my baby really the size you say each week?

Each of our weekly fetal development pages has information about your unborn baby's size and weight. These are average figures.

All babies are about the same size early in pregnancy, but they begin to grow at different rates during the second trimester. That's why some full-term babies weigh less than 2.5kg (5.5lb) and some weigh more than 3.9kg (8.8lb) (Hill 2019).

Try not to worry too much if an ultrasound scan indicates that your baby is not the average size. As your pregnancy progresses, the range of what's considered a normal size grows larger. Your doctor or midwife will let you know if they have any concerns about the size of your baby.

How do I switch from getting pregnancy to baby newsletters each week?

If you've had your baby recently and you're still receiving BabyCentre newsletters about being pregnant, or if you're just getting the newsletter for the wrong week, all you need to do is update your family details.

• Either use the link above or click on your profile picture in the top right corner of the screen, then select ‘My profile’ and click on edit my family details. This will take you to the family details page.
• You can then remove or change your due date or birth date information.
• When you've changed your due date or your baby's birth date, click on "Save changes".

References

Hill MA. 2019. Birth weight. University of New South Wales Embryology. embryology.med.unsw.ed.au. [Accessed December 2019]

Mongelli M. 2016. Evaluation of gestation. Medscape. emedicine.medscape.com [Accessed December 2019]

NHS. 2017. The pregnancy dating scan. NHS, Health A-Z, pregnancy and baby. www.nhs.uk [Accessed December 2019]

NHS. 2018a. You and your baby at 1 to 3 weeks pregnant. NHS, Health A-Z, Pregnancy and baby. www.nhs.uk [Accessed December 2019]

NHS. 2018b. You and your baby at 38 weeks pregnant. NHS, Health A-Z, Pregnancy and baby. www.nhs.uk [Accessed December 2019]

NHS. 2018c. You and your baby at 40 weeks pregnant. NHS, Health A-Z, Pregnancy and baby. www.nhs.uk [Accessed December 2019]

NHS Digital. 2019. NHS maternity statistics, 2018-19. Hospital Episode Statistics NHS maternity statistics tables. digital.nhs.uk [Accessed December 2019]

NICE. 2008. Antenatal care for uncomplicated pregnancies. National Institute for Health and Care Excellence, Clinical guideline CG62. www.nice.org.uk [Accessed December 2019]

Papageorghiou AT, Kennedy SH, Salomon LJ, et al 2014. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy. Ultrasound Obstet Gynecol 44(6):641-8. obgyn.onlinelibrary.wiley.com [Accessed December 2019]

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