Stages of pregnancy

Published by Bupa's Health Information Team, April 2010.

This factsheet is for women who would like information about how their body will change and how their baby will develop during pregnancy.

Animation - How a baby develops during pregnancy

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Trimesters

A normal pregnancy lasts around 37 to 42 weeks, or an average of 40 weeks from the first day of your last period. Pregnancy is often divided into thirds, called trimesters. It's a useful way to help describe the changes that happen during pregnancy. The first trimester is week one to week 12, the second is week 13 to week 27, and the third is week 28 to week 40.

Conception

One of your ovaries releases an egg around 14 days before your period. This is called ovulation. It's usually in the middle of a 28-day menstrual cycle, but can be later if you have longer cycles and earlier if your cycle is shorter. The egg can be fertilised by sperm in one of your fallopian tubes. Conception is when the fertilised egg then travels along the tube and implants in your womb (uterus). Implantation happens about six days after fertilisation. The fertilised egg grows and is called an embryo.

When you become pregnant, your womb doesn't shed its lining as it normally does at the end of a menstrual cycle, so you don't have a period each month. A missed period is one of the early signs of pregnancy.

You can buy a pregnancy test from a pharmacy or supermarket. These can test for pregnancy from the first day of a missed period and, if the test is positive, then it indicates that you are pregnant. Your GP or family planning clinic can also arrange a pregnancy test for you. Some testing kits can give a result even before you miss a period.

First trimester (one to 12 weeks)

You

During the first trimester, the changes in your hormones cause various changes to your body.

As well as a missed period, you might have other early symptoms of pregnancy, including breast tenderness. You may also need a larger, more supportive bra. Your nipples and the surrounding area (areola) may become larger and darker. You may need to pass urine more often because your growing womb starts to press on your bladder, and the hormonal changes may make you feel very tired.

Many women feel sick, and some actually are sick in early pregnancy. This is called 'morning sickness' but it can happen any time or throughout the day. The cause is unknown, but some research suggests it may be related to high levels of pregnancy hormones. There are things you can do to try and reduce the nausea and sickness: get plenty of rest, eat little and often, and avoid smells and tastes that make you nauseous. You should see your GP or midwife if you can't keep any food or fluid down.

You may become constipated because the rising hormone levels slow your bowels down. To relieve this, drink plenty of fluids and eat a healthy diet with plenty of fibre. Heartburn is also common.

Take 400 microgram (0.4mg) folic acid supplements every day for the first 12 weeks of your pregnancy. This is when the baby's brain and nervous system develop. Folic acid reduces the chance of the baby being born with a spinal cord problem called a cord neural tube defect (NTD), such as spina bifida. If you're at a high risk of having a baby with a NTD, you should take a dose of 5,000 micrograms (5mg) folic acid daily.

It's best not to drink alcohol during pregnancy. This is particularly important during the first three months, when important organs are forming. If you do choose to drink alcohol, stick to no more than one or two units of alcohol, just once or twice a week, and don't get drunk.

Your baby

Your baby will grow and develop quickly in the first trimester. By the time you are 10 weeks pregnant (eight weeks after fertilisation) all the body parts are present (if not quite fully developed) and the placenta is fully formed. At this point, the embryo is known as a fetus. Nutrients are transferred from you to the baby through the placenta. Waste products from the baby are returned to your circulation to be removed.

By the time you are 12 weeks pregnant (10 weeks after fertilisation), your baby is approximately 60mm long. At this stage, your baby's forehead grows with the developing brain, the eyelids are still sealed shut and the ears are forming.

Second trimester (13 to 27 weeks)

You

Your pregnancy will start to show during the second trimester - when this happens varies from woman to woman.

Any sickness or nausea usually gets better by the time you are 16 to 20 weeks pregnant. You may get backache, hip pain or pelvic pain because pregnancy hormones cause your ligaments and tendons to relax, and your posture may change as your baby grows.

You may first feel the baby move around 18 to 20 weeks of pregnancy, but this varies between women and may be a few weeks earlier if it's not your first pregnancy. The movements become much more vigorous and obvious as the baby gets bigger and stronger.

Your baby

In the second trimester, your baby's organs mature and its skeleton starts to harden. The baby swallows amniotic fluid and passes it out through its gut. The kidneys start to work and pass small amounts of urine.

By 19 weeks your baby is able to hear, and is covered in fine hair called lanugo. If you have a scan at this stage, it's often possible to find out whether the baby is a boy or a girl. By the end of the second trimester, your baby has a chance of surviving if he or she is born prematurely, but would need intensive care.

Third trimester (28 to 40 weeks)

You

As your abdomen and breasts grow, you may get stretch marks. These are harmless and usually fade after the baby is born. You may find the extra weight you are carrying makes you tired, and you may get breathless as your expanded womb makes your lung capacity smaller.

Some women have trouble finding a comfortable position to sleep in. Your baby's head drops down into your pelvis (engages) ready for delivery - for a first baby this is usually towards the end of the pregnancy (around 37 weeks onwards). This can cause pressure on your bladder, and you might need to pass urine more often. The head may not engage until labour if you have had a baby before.

Braxton Hicks contractions - practice contractions - can start in the second trimester, but are much more likely in the third. These can be mistaken for labour - but the difference is that real labour contractions are regular. Braxton Hicks contractions last only about 25 seconds.

Your baby

Your baby's lungs mature throughout the third trimester, and by 32 weeks he or she is much more likely to survive if born prematurely. Your baby makes breathing movements, even though the lungs don't work properly until birth. Fat stores are laid down in preparation for birth. Your baby grows fine hair and fingernails, the eyes open and close, and teeth may start growing under the gums.

Weight gain in pregnancy

You will put on weight during your pregnancy - the exact amount varies from woman to woman. Your midwife will weigh you at your first appointment. Unless you are very overweight or very underweight, your midwife probably won't weigh you again because it doesn't necessarily give any useful information about your developing baby.

If you're concerned about your weight gain, talk to your midwife or GP. He or she will be able to tell you whether it is too much or too little.

The weight you gain during pregnancy isn't the same as 'getting fat'. The extra weight is made up of:

• the developing baby, placenta and amniotic fluid
• the growth of your womb and breasts
• the increased blood in your circulation
• water retention
• essential fat stores

For answers to frequently asked questions on this topic, see Common questions.

For sources and links to further information, see Resources.