Trimesters and months of pregnancy

Pregnancy in weeks, months and trimesters

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.

Tips for a healthy pregnancy

Do you know how to keep in tip-top health throughout your pregnancy? Here are five tips on staying healthy. More pregnancy videos

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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Pregnancy Week By Week | First Month Symptoms and Signs

In This Section

• Month by Month
• What happens in the second month?
• What happens in the third month?
• What happens in the fourth month?
• What happens in the fifth month?

• What happens in the sixth month?
• What happens in the seventh month?
• What happens in the eighth month?
• What happens in the ninth month?
• What happens in the tenth month?

What happens in the first month of pregnancy?

Pregnancy is divided into 3 trimesters. Each trimester is a little longer than 13 weeks. The first month marks the beginning of the first trimester.

What’s gestational age?

Pregnancy timing is measured using “gestational age.” Gestational age starts on the first day of your last menstrual period (LMP).

Gestational age can be confusing. Most people think of pregnancy as lasting 9 months. And it’s true that you’re pregnant for about 9 months. But because pregnancy is measured from the first day of your last menstrual period — about 3-4 weeks before you’re actually pregnant — a full-term pregnancy usually totals about 40 weeks from LMP — roughly 10 months.

Many people don’t remember exactly when they started their last menstrual period — that’s OK. The surest way to find out gestational age early in pregnancy is with an ultrasound.

What happens during week 1 - 2?

These are the first 2 weeks of your menstrual cycle. You have your period. About 2 weeks later, the egg that’s most mature is released from your ovary — this is called ovulation. Ovulation may happen earlier or later, depending on the length of your menstrual cycle. The average menstrual cycle is 28 days.

After it’s released, your egg travels down your fallopian tube toward your uterus. If the egg meets up with a sperm, they combine. This is called fertilization. Fertilization is most likely to occur when you have unprotected vaginal sex during the 6 days leading up to — and including the day of — ovulation.

What happens during week 3 - 4?

The fertilized egg moves down your fallopian tube and divides into more and more cells. It reaches your uterus about 3–4 days after fertilization. The dividing cells then form a ball that floats around in the uterus for about 2–3 days.

Pregnancy begins when the ball of cells attaches to the lining of your uterus. This is called implantation. It usually starts about 6 days after fertilization and takes about 3–4 days to be complete.

Pregnancy doesn’t always happen, even if an egg is fertilized by a sperm. Up to half of all fertilized eggs pass out of your body when you get your period, before implantation is complete.

What are the signs of pregnancy?

For a lot of people, the first sign of pregnancy is a missed period. Most pregnancy tests will be positive by the time you’ve missed your period. Other early pregnancy symptoms include feeling tired, feeling bloated, peeing more than usual, mood swings, nausea, and tender or swollen breasts. Not everyone has all of these symptoms, but it’s common to have at least 1 of them.

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weekly pregnancy calendar. At first sight - step by step to children's happiness

Hurrah! The pregnancy test came back positive and the doctor just confirmed the result. Now it's time to wait. Nine months is a long time. But thanks to our calendar, you can find out now and at any time what happens to you and your baby in each of the weeks of pregnancy.

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40 weeks of pregnancy: calculated from the date of the last menstruation

Doctors prefer to calculate in weeks: pregnancy lasts 40 weeks. But is it so? Isn't pregnancy 38 weeks long? Both options are correct. Many women are confused by the fact that weeks from the date of the last period are already counted, even if the pregnancy has not actually occurred yet. Why? Counting from the beginning of the cycle, it is easier for the doctor to make a more accurate assessment of the stage of development of the child and plan the date of his birth. The pregnancy calendar starts on the first day of your last period. This is the starting point for 40 weeks.

You will most likely notice that you are expecting a baby on the day your period stops. So, you are already in the 5th week of pregnancy!

Weekly calendar: follow the progress of your pregnancy

In addition to being divided into weeks or months, pregnancy is also traditionally divided into three main stages, the so-called trimesters of pregnancy. Here is an overview of the changes in your body and the development of your baby by trimester, month and week.

1st trimester of pregnancy (weeks 1-12): the body must undergo changes

• Mom in the first trimester: how your body changes during weeks 1-12 of pregnancy

Your body has a lot to do to adjust to pregnancy. Hormones are working at full capacity. This leads to many pregnancy symptoms. You may feel sick and tired, and your breasts become hypersensitive. Everything changes:

• Teeth: your body produces less saliva, making your teeth more prone to cavities and gingivitis.

• Skin: Increased estrogen levels reduce sebum production - many pregnant women experience clearer skin and smooth out fine lines. However, you may be more sensitive to the sun, which is why freckles and skin spots become darker.

• Hair: they become darker, and the hormone progesterone slows down hair loss. In many pregnant women, hair becomes more shiny and voluminous.

• Breast: The mammary glands develop and the breasts increase in size. Areola and nipple become darker. This change makes the nipple less sensitive. Montgomery's glands, located on the areola, may increase in size, forming small tubercles. They secrete a secret that protects the breast from infection, cares for the skin and, after birth, directs the baby to the nipple with its scent.

• In the first trimester, you may not want to tell everyone about your pregnancy yet, because the chances of miscarriage are higher in the first three months. Once you cross the threshold of 12 weeks, you have already taken a big step. This means that at present the pregnancy is proceeding quite steadily, and the child is well settled in the womb. From this point on, the risk of miscarriage is significantly reduced.

  Baby in the first trimester: how your baby develops from 1 to 12 months of pregnancy

  First month of pregnancy: after fertilization, cell division begins, and every day the embryo consists of more cells. The first nerve cells are formed around the 18th day. At 4 weeks, the embryo develops.

  Second month of pregnancy: at the beginning of the second month, the embryo is about 2 mm in size. At the sixth week, the size of the embryo is already about 4 millimeters, and from the 7th week it grows by about one millimeter per day. The embryo lies protected in the amniotic sac, its limbs are attached, and it produces its own blood. The heart begins to beat and can be heard on an ultrasound no later than the 8th week.

  Third month of pregnancy: at the last stage of embryonic development, the head has grown and acquired human features. Thanks to the development of the brain, the fetus can already move all parts of the body. At week 12, the fetus already has a height of about five to six centimeters. At this time, all organs are created.

  This is the size of your baby in the first trimester: Ø approx. 5 cm.

2nd trimester of pregnancy (weeks 13-28): really beautiful pregnancy

Mom in the second trimester: how your body changes from 13 to 28 weeks of pregnancy

In general, you are doing well. Your body has adapted to pregnancy. You are in good physical shape and have a great appearance. Gradually, your clothes no longer fit. It's time to buy comfortable pants! Pregnancy is already visible from the outside.

• Well-being: symptoms of pregnancy such as nausea and fatigue are getting better.
• Body shapes: your waist is slowly disappearing. In addition to stretch marks, a vertical dark line may appear in the middle of the abdomen - "linea nigra". Water retention can cause swelling, especially in the lower extremities (such as the feet).
• Teeth: increased blood flow causes the gums to swell, often resulting in bleeding gums.
• Breasts: Since most women's breasts begin to produce milk around the 16th week of pregnancy, a drop may leak from time to time. In general, the breasts increased in size. Your old bras are too small and too tight.

Tip: is better to buy a good bra that grows with you. A good model is, for example, the elastic Medela maternity and nursing bra. It is not only comfortable to wear, but also adapts to the changing shape of the breast. When the time comes, the cups can be simply opened with a single movement of the breastfeeding clasp.

Second trimester baby: how he or she develops at 13 to 28 weeks

4 months pregnant: by the end of the first trimester all organs appeared, and the critical phase of development ended. The reproductive organs of the fetus develop, the intestinal system matures, and the proportions of the head and body gradually level off.

Height/weight at the end of the 16th week: Ø about 11 cm/150 g. At the beginning of the 18th week of pregnancy, the fetus is so mobile that you can feel it move. Your baby's gender is now likely to be by ultrasound . The baby reacts to external sounds and to your mood. Since emotional hormones are also carried across the placenta, they cause similar emotions in your baby as well.