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Pregnancy at weeks 1 to 4
Your baby
Your pregnancy began when your egg (ovum) was fertilised by sperm. Although your pregnancy and due date, is typically calculated according to the first day of your last period, before conception has even happened.
Over the next 4 or 5 days, your fertilised egg continues to divide and travel down the fallopian tube towards your uterus.
By the end of the fourth week, the embryo implants itself into your uterus. It is now called a blastocyst. It is about 0.2mm wide and contains about 200 cells that are continuing to multiply.
The embryo is made up of different layers of cells:
- The outer layer, will become the baby's nervous system and brain.
- The middle layer, will become the heart, blood vessels, muscles, and bones.
- The inner layer will become the breathing and digestive systems.
At this early stage the outside of the embryo has small tentacles called chorionic villi that reach out to form links with you blood supply. This will develop into the placenta over the coming weeks, supporting your babies oxygen and nutrients needs.
Your body
You probably won't notice any signs that you're pregnant just yet.
The first sign for many women is that they miss their period at the end of week 4. If you've noticed a missed period, you might be feeling all kinds of different emotions, from happiness and excitement, to worry and fear. Rest assured that all these feelings and more are totally valid and normal.
Although you may not have noticed any changes just yet, as soon as you conceive, the hormone levels in your body start to change. You produce more of the hormone progesterone, which prevents you from having a period, and there is an increase in the level of human chorionic gonadotropin (hCG). This is the pregnancy hormone that is detected when you do a pregnancy test.
Things to remember
Your baby will have the best possible start in life if you are healthy when you conceive.
If you are trying for a baby, it's a good idea to take 400 micrograms of folate every day to prevent neural tube defects such as spina bifida. If the baby is planned, you might consider seeing your doctor to talk about genetic screening or to be tested for any sexually transmitted infections before you fall pregnant.
You should try to eat a healthy diet and exercise regularly. Not drinking alcohol and giving up smoking are also best for a healthy pregnancy and baby.
Resources and support
Speak to your doctor, midwife or obstetrician if you have questions about your pregnancy.
Pregnancy, Birth and Baby also has more information on:
- maternity care in Australia
- having a healthy pregnancy
- exploring the anatomy of pregnancy and birth
- checkups, screenings, scans and tests you can have during your pregnancy
- common conditions you might experience during your pregnancy
- looking after your mental health and wellbeing
Find out how we develop and review our content.
Last reviewed: August 2023
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