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Stages of pregnancy
Published by Bupa's health information team, March 2008.
This factsheet is for women who would like information about how their body will change during pregnancy.
How a baby develops during pregnancy
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A normal pregnancy lasts around 280 days, or 40 weeks from the first day of your last period. Pregnancy is often divided into thirds, called trimesters. The first trimester is week one to week 12, the second is week 13 to week 26, and the third is week 27 to week 40.
Your ovary 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 is fertilised by the sperm in one of your two fallopian tubes. Conception is when the fertilised egg then travels along the tube and implants in your womb (uterus). 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. Your GP or family planning clinic can also do a pregnancy test for you.
The baby and surrounding structures in late pregnancy
First trimester (one to 12 weeks)
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 signs of pregnancy, including breast tenderness. You may also need a larger, more supportive bra. At around 12 weeks your nipples and the surrounding area (areola) may become larger and darker. You may need to pass urine more often. You might also feel very tired, especially at 12 to 15 weeks.
Around four out five women feel sick, and around half are sick in early pregnancy. This is called "morning sickness" but it can happen at any time of day. High levels of pregnancy hormones may be the cause. Eating small, regular meals and avoiding spicy and greasy foods can help. 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. Prevent or relieve this by drinking plenty of fluids and eating 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, such as spina bifida.
During your pregnancy, it's important not to take medicines or recreational drugs that could affect your baby's development. Ask your GP or pharmacist for advice. 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 decide to drink alcohol, stick to no more than one or two units of alcohol, once or twice a week and don't get drunk.
Your baby will grow and develop quickly in the first trimester. Most of the organs, including the nervous system and heart, are formed by the time you are nine weeks pregnant (seven weeks after fertilisation). At this point the embryo is known as a fetus.
The placenta is fully formed by the time you are around 10 weeks pregnant (eight weeks after fertilisation). 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 neck is uncurling and the limbs are complete. The eyelids are still fused but ears are forming.
Second trimester (13 to 26 weeks)
Your sickness and nausea should be improving by the time you are around 14 weeks pregnant. You may get backache because pregnancy hormones cause your ligaments and tendons to relax, and the weight of the baby may cause you to arch your back.
You may first feel the baby move around 17 to 20 weeks of pregnancy, but this varies between women. The movements become much more vigorous and obvious as the baby gets bigger and stronger. Braxton Hicks, or practice contractions, are painless tightenings of the womb that may start from around 20 weeks of pregnancy.
In the second trimester your baby's sex organs develop and other organs mature. The baby swallows amniotic fluid and passes it out through its gut. The kidneys start to work and pass small amounts of urine.
Your baby can now hear, and is covered in fine hair called lanugo. By the end of the second trimester your baby would have a chance of surviving if he or she was born, but would need intensive care.
Third trimester (27 to 40 weeks)
As your abdomen and breasts grow, you may get stretch marks. These will fade after the baby is born although they don't disappear altogether. 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. The baby's head drops down into your pelvis (engages) ready for delivery - for a first baby this is 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 become stronger in the third trimester.
Your baby's lungs mature throughout the third trimester. The baby makes breathing movements, even though the lungs don't work properly until birth. Fat stores are laid down in preparation for birth. The 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 and also depends on your weight before you become pregnant. You can expect to gain around 10 to 12.5kg (22 to 28lbs) during pregnancy if you are a healthy weight to start with. Your GP or midwife may have specific advice for you if you weigh less than 50kg (7st 12) or more than 100kg (15st 10).
The weight you gain during pregnancy is made up of:
the developing baby, placenta and amniotic fluid
the growth of your womb and breasts
the increased blood in your circulation
- Arulkumaran S, Symonds I M, Fowlie A. Oxford Handbook of Obstetrics and Gynaecology. Oxford University Press: 2004: 60
- Oats J, Abraham S. Fundamentals of Obstetrics and Gynaecology. 8th ed. Mosby 2005
- Simon C, Everitt H, Kendrick T. Oxford Handbook of General Practice. 2nd ed. Oxford University Press: 2006: 778
- Emma's Diary. Royal College of General Practitioners. www.emmasdiary.co.uk, accessed 19 March 2007
- Moffett D, Moffett S, Schauf C. Human Physiology: Foundations and frontiers. Mosby 2nd edition 1993: 713
- Pregnancy diagnosis. GP Notebook. www.gpnotebook.co.uk, accessed 6 February 2007
- Antenatal care: routine care for the healthy pregnant woman. National Institute of Health and Clinical Evidence. 2003. www.nice.org.uk, accessed 6 March 2007
- The pregnancy book. 2007 edition. Department of Health. www.dh.gov.uk
- Trying for a baby. Eat well, be well. www.eatwell.gov.uk, accessed 17 March 2007
This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.
Publication date: March 2008. Expected review date: March 2010.