Watch your unborn baby grow week by week – 1st Trimester of your pregnancy

From the early signs of pregnancy follow your baby's growing fetal development in this week by week guide with pictures through your 1st trimester


Your pregnancy week by week

Congratulations, you’re pregnant!  Here you can discover all the amazing things that are happening inside your womb while you’re waiting for your baby to be born.


The First Trimester of your pregnancy

This is the first 12 weeks of pregnancy and a crucial period in terms of your baby’s development. By the end of the 12th week of pregnancy, all your baby’s major organs and body systems will be formed. During this time you might have worries over things like pregnancy bleeding, want to know when it’s safe to announce the news of your baby and calculating your due date. But mostly you’re going to want to know all about what’s going to happen in the next 40 weeks of pregnancy, particularly how your baby is going to look. 

*Remember, all pregnancies are different and foetal growth rates vary, this is meant only as an approximate guide to development. If you have any concerns about your developing pregnancy then speak to your doctor.


Pictures of your baby inside your tummy two weeks

The 40 weeks of pregnancy start from the last day of your period. This means ‘week 2’ is around the point at which you conceive, because you’re most fertile in the middle of your cycle, two weeks after your last period. 

Your egg and his sperm each have 23 chromosomes and together they form the 46 chromosomes which make up a human cell, the zygote. It splits into two cells, each with 46 chromosomes, then these divide again and again as they pass down the fallopian tube. By the time they reach the uterus there are about 100 cells – the building blocks for your baby. 


Pictures of your unborn baby at three weeks of your pregnancy

This is a crucial week in your baby’s development. The mass of cells which has been steadily dividing since fertilisation is called the ‘blastocyst’ and is now ready to burrow its way into your womb lining (endometrium). This is called ‘implantation’ and can be one of the most tricky points in early pregnancy. Implantation can lead to something called Implantation bleeding which can be a cause for concern. Once the blastocyst has implanted then the embryo can receive the oxygen and nutrients from you to be able to grow.


What’s happening inside your pregnancy tummy at four weeks

Now your baby-to-be is called an embryo, and is around the size of a pin head. The cells are dividing and multiplying rapidly, and three layers of cells have now formed – the outer layer (the ectoderm), which will develop into your baby’s brain, nervous system, skin, hair, nails and teeth; the middle layer (the mesoderm), which will become her heart and blood vessels, bones, muscles and reproductive organs; and the inner layer (the endoderm), which will develop into her lungs, liver, bladder and digestive system. The cells of the embryo grow lengthwise, assuming a leaner shape. 

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