Stages of pregnancy

Expert reviewer, Dr Samantha Wild, General Practitioner, Bupa UK and Michelle Sheridan, Midwife, Bupa UK
Next review due August 2023

Pregnancy usually lasts from 37 to 42 weeks, counting from the first day of your last period. It’s divided into three trimesters. Each trimester lasts for three months (around 12 to 13 weeks). During pregnancy, your body goes through lots of changes to support your growing baby and get ready for the birth.

Getting pregnant

Getting pregnant means a sperm from your male partner joins together with (fertilises) one of your eggs. This is also called conception.

Every month, one of your ovaries releases an egg. This is called ovulation. It usually happens 10 to 16 days after the start of your last period. If you’ve had sex, the egg may be fertilised by sperm from your partner in your fallopian tube. The fertilised egg then moves along your fallopian tube towards your womb (uterus). It quickly grows and divides, becoming a ball of cells.

Once it reaches your womb, the ball of cells buries itself into the womb lining. This is called implantation. It usually happens around six days after conception. At this early stage, your baby is called an embryo. Usually, your womb sheds its lining every month, causing your period. This happens around 14 days after you’ve ovulated. When you’re pregnant, your womb won’t shed its lining, so you’ll stop having periods. Missing a period is often the first sign that you’re pregnant.

Pregnancy tests

You can buy pregnancy tests at any pharmacy or supermarket. You may also be able to get one free of charge from certain charities, contraceptive and sexual health clinics or your GP.

Pregnancy tests work by detecting a pregnancy hormone called beta-human chorionic gonadotrophin (hCG) in your urine. Most pregnancy tests can detect if you’re pregnant from the first day of your missed period. But some tests are more sensitive and can give you a result even earlier than this.

Pregnancy tests are usually very accurate. But sometimes you can get a negative result if you do the test too early. If you get a negative result but still think you’re pregnant, it’s worth doing the test again after a few days. During the first few weeks of pregnancy, your level of hCG increases very quickly. By the time your period is about a week late, you can be pretty sure that the result of a pregnancy test is accurate.

How many weeks pregnant am I?

You count your pregnancy from the first day of your last period. So, if you have a 28-day menstrual cycle, you’re said to be four weeks pregnant on the day your next period would have been due. If it’s been a week since your period was due, you’ll be five weeks pregnant.

Your doctor or midwife will calculate your due date based on the date you started your last period. Your due date is 40 weeks from this date. But it’s normal to give birth up to three weeks before or two weeks after your due date.

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First trimester (one to 12 weeks)

Effects of pregnancy on you

Your body goes through lots of changes in early pregnancy.

  • Missing your period is often the first sign that you’re pregnant.
  • Your breasts may feel fuller and tender. Your nipples and the area around them (the areola) may get darker.
  • You may find you need to pee more often, as your womb gets bigger and starts to press on your bladder.
  • You may feel very tired and sleepy.
  • You may get pulling pains around the sides of your tummy, as your womb grows and stretches the ligaments around it.
  • You may feel sick or be sick. This is often called ‘morning sickness’, but it can happen at any time of the day. It’s very common early on in pregnancy. You may find your symptoms get better if you eat little and often and stick to blander foods. If your sickness is severe, you may have hyperemesis gravidarum (HG), and might need hospital treatment.
  • You may get constipation because food passes more slowly through your digestive system. Make sure you’re drinking lots of fluids and getting plenty of fibre in your diet.


Many of these symptoms will go away as your pregnancy continues. But some things – for example, constipation – may carry on or get worse. If you have any pregnancy problems, it’s important to speak to your GP or midwife.

Before and during your pregnancy, keep yourself as healthy as possible. You may need to make some changes to your lifestyle. These changes may include:


How your baby develops

Your baby begins life as a tiny ball of cells which grows quickly. Some of the cells become an embryo and some form the placenta, which creates a lifeline between you and your baby. The placenta attaches itself to your womb and is linked to your baby by the umbilical cord. Oxygen, nutrients and hormones from your blood pass through the placenta and umbilical cord into your baby. Your baby’s waste products pass back into your blood, so you can get rid of them.

During these first few weeks, all your baby’s organs and important body parts start to form. The cells that create your baby’s heart make beating movements from very early on. Your baby’s heartbeat can usually be detected during an ultrasound scan around the sixth week of pregnancy.

By eight weeks, your baby’s lungs are already forming the tubes that will carry air in and out once they’re born. The muscles of their eyes, nose and mouth are forming, and their fingers and toes start to poke out of their hands and feet. By week 12, your baby will even have tiny fingernails. Their kidneys now start to make urine and their pancreas starts to make insulin, which will control their blood sugar.

Second trimester (13 to 27 weeks)

Effects of pregnancy on you

In your second trimester, your body continues to change as your baby's organs continue to grow and mature.

  • Most people will notice you’re pregnant once you’ve reached around the 20th week of pregnancy. Lots of other changes happen in your second trimester too. You’ll start to put on more weight as your baby grows. Every pregnancy is different, but you may put on around 10–12.5kg (22–27.5 pounds) during your pregnancy. This extra weight is made up of your growing baby, the placenta, the fluid in your womb, your breasts and extra fluid and fat stores.
  • As your tummy and breasts grow, you may notice stretch marks on your skin. These are harmless and usually fade after your baby is born. You can try different moisturisers and lotions to prevent stretch marks but there’s little evidence that any of these works.
  • Morning sickness usually gets better by the time you’re 16 weeks pregnant.
  • Pregnancy hormones make your ligaments and tendons relax, which loosens your joints. So, you may get backache. This can also be caused by the extra weight you’re carrying. You may get pain in your pelvis and find it harder to walk. You may be offered some physiotherapy to help manage this.
  • You may first feel your baby move when you're around 18 to 20 weeks pregnant. This might happen a few weeks earlier if this isn’t your first pregnancy. The movements will get stronger and more obvious as your baby gets bigger and more active. Make time to notice your baby’s movements and get to know their usual pattern. Contact your midwife of antenatal ward if you notice any changes (reduced movement).


How your baby develops

By 16 weeks, your baby’s bones harden and by 20 weeks, their digestive system is working. Your midwife will be able to hear your baby’s heartbeat with a stethoscope from about 16 to 20 weeks. Their ears develop and by 16 weeks, your baby can hear some sounds.

Your baby’s skin will be wrinkled and covered with a greasy substance and fine hair called lanugo. They’ll sleep and wake regularly and may start to kick and stretch. Their sucking reflex is developing, and they may even suck their thumb if their hand reaches their mouth. Their eyelids will open by the time they’re about six months. By about the 14th week of pregnancy, the sex of your baby can usually be seen in an ultrasound scan.

From 24 weeks, your baby starts to have more chance of surviving if they’re born prematurely. But they would need intensive care for a long time if they were born this early. Their chances of surviving get better with every week they stay inside your womb.

Third trimester (28 to 40 weeks)

Effects of pregnancy on you

In your third trimester, your baby keeps growing and your body gets ready for the birth.

  • The extra weight you’re carrying can make you very tired, and you may feel breathless as your baby grows. Sometimes, this may be caused by iron-deficiency anaemia. You should be offered blood tests to check your iron levels at your first antenatal appointment and again at 28 weeks.
  • You may be finding it hard to sleep. This may be because you wake up to wee in the night or you can’t get into a comfortable sleeping position.
  • You may get Braxton–Hicks contractions – where your womb starts to contract – from about 30 weeks. These may be uncomfortable but shouldn’t hurt.
  • You may get heartburn. Try to eat little and often and stick to bland foods. Avoid eating late at night. You can try an antacid medicine to see if it helps.
  • You may notice swelling later in pregnancy, especially in your ankles, feet, legs, hands or fingers. If this happens suddenly or gets a lot worse, contact your midwife. It can be a sign of pre-eclampsia (high blood pressure during your pregnancy).
  • You may get varicose veins, as the blood flow around your legs slows down.
  • Your breasts may start to leak a milky fluid called colostrum.
  • You may need to wee more often as your baby drops into your pelvis. You may leak some urine when you cough or sneeze. This is called stress incontinence. Doing pelvic floor exercises can help with this.


How your baby develops

During the third trimester, your baby puts on weight quickly and stores fat under their skin. Their skin starts to smooth out and they lose their coat of fine hair. Their body gradually gets more in proportion. Their lungs are fully formed but won’t work properly until they’re born.

Your baby will be making lots of movements, including stretching, kicking and grasping. And they’ll startle at loud sounds and sense changes in light. They usually turn into a head-down position by week 36, ready for birth. In the last few weeks, they tend to drop lower into your pelvis.

An image showing the baby and surrounding structures in late pregnancy

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  • Reviewed by Victoria Goldman, Freelance Health Editor and Alice Windsor, Specialist Health Editor, Bupa Health Content Team, August 2021
    Expert reviewer, Dr Samantha Wild, General Practitioner, Bupa UK and Michelle Sheridan, Midwife, Bupa UK
    Next review due August 2024