Your Baby’s Development: Key Changes Across the Weeks of Pregnancy

Weekly information can make pregnancy feel more tangible. It helps to read it as a guide to a sequence of changes, while using your own appointments and scans for information about your baby.

How pregnancy weeks are counted

Pregnancy is usually dated from the first day of the last menstrual period, rather than from fertilisation. A scan can help establish the estimated due date when dates are uncertain.

The guide to pregnancy with uncertain dates explains questions about dating. Keep the date your maternity team uses in your notes. Ask them to explain any adjustment so you know which week to follow.

The early weeks

During the first trimester, major structures begin forming, including the brain, spine, and developing organs. Hands and feet take shape. By weeks 9 to 12, features such as eyelids and fingernails are developing.

You may want to write down one question for your next appointment instead of trying to remember every detail in a weekly guide.

The middle and later weeks

The second trimester brings continuing growth, bone development, and increasingly noticeable movement. Later, the baby gains more weight and body fat while organs continue maturing, including the lungs and nervous system.

Size comparisons are illustrations, not a measurement of your baby. If a comparison worries you, ask what your own scan or growth assessment shows.

Use the guide to prepare questions

At each stage, ask what appointments or tests are coming next and what they are intended to check. Tell your team if you are unsure about the explanation of a result.

Weekly guides do not replace assessment of symptoms. If your baby moves less, stops moving or changes their usual pattern, contact your midwife or maternity unit immediately, including at night. Do not wait for a new week or appointment. Read the guidance on baby movements for more detail.