Pregnancy is divided into three trimesters, with each stage bringing changes to the pregnant person’s body and the developing baby. Although pregnancy is often described week by week, symptoms and their timing can vary from one person to another.
The first trimester generally covers weeks 1 through 12. The second runs from about week 13 to week 27, while the third begins around week 28 and continues through birth. The NHS describes weeks 4 to 12 as the first trimester, weeks 13 to 27 as the second and weeks 28 to 41 as the final trimester.
First trimester
Early pregnancy symptoms commonly begin around four to six weeks of pregnancy, although not everyone experiences them. Common symptoms include a missed period, nausea or vomiting, tiredness, sore breasts, increased urination, constipation, bloating and changes in food preferences or sensitivity to smells.
At around the fourth week, a person may begin noticing symptoms such as tiredness, nausea or more frequent urination. By the 10th week, bloating, indigestion, headaches, dizziness and cramping can also occur alongside earlier symptoms.
Symptoms alone cannot confirm a pregnancy. A pregnancy test is used to establish whether someone is pregnant, and prenatal care should begin after pregnancy is confirmed.
The first trimester is also an important period of fetal development. By 12 weeks, the baby’s organs and muscles have developed, while the placenta is increasingly taking over its role in supporting the pregnancy.
Second trimester
The second trimester begins at about 13 weeks. For many people, some early pregnancy symptoms become less intense during this period, although other symptoms can develop as the pregnancy progresses.
Backache, headaches, indigestion, heartburn, constipation, leg cramps, dizziness and swelling are among the symptoms that can occur during the second trimester. Some people may also experience round ligament pain as the uterus expands.
The second trimester is also a period of continued fetal growth and development. Prenatal appointments remain important because healthcare professionals monitor the health of the pregnant person and the developing baby.
Third trimester
The third trimester begins around week 28 and continues until birth. As the pregnancy progresses, physical symptoms can include backache, heartburn, swelling, difficulty sleeping and leg cramps.
Some people may also experience Braxton Hicks contractions, which are usually painless contractions that can occur before labour. At 38 weeks, for example, the NHS lists Braxton Hicks contractions among common symptoms of late pregnancy.
Pregnancy does not always end at exactly 40 weeks. The final weeks can bring further physical changes as the body prepares for labour. At 40 weeks, symptoms may include lower backache and contractions, although these do not necessarily mean labour has begun.
When to seek medical attention
Vaginal bleeding during pregnancy should not automatically be assumed to be a normal sign of pregnancy. It can have several causes, including changes in the cervix, infection, miscarriage, ectopic pregnancy or problems involving the placenta. Bleeding after sex can also occur because pregnancy-related changes to blood vessels in the cervix can make them more likely to bleed.
Anyone who experiences bleeding during pregnancy should seek medical advice, particularly if it is accompanied by pain or other concerning symptoms. Heavy bleeding, severe abdominal pain, dizziness, fainting or shoulder pain can require urgent medical attention.
Other symptoms can also require prompt assessment. For example, sudden vision problems or sudden swelling of the face, hands, legs or feet can be associated with conditions such as pre-eclampsia and should be checked by a healthcare professional.
Pregnancy develops differently from one person to another, so a week-by-week guide provides a general framework rather than a prediction of exactly how every pregnancy will progress. Regular prenatal care allows healthcare professionals to monitor the pregnancy and address symptoms or complications when they arise.
