First Trimester

Your changing body and your growing baby

Babies develop at their own unique pace and every pregnancy is different, below is a guide to the progression of a normal pregnancy.

Your first trimester

Baby’s development

Weeks 1-4

The fertilised egg moves through the fallopian tube and reaches the uterus after three to four days where it will eventually attach to the lining of the uterus or womb.

During this time it undergoes a process of division and forms a cluster of cells, this will develop into a baby and the amniotic fluid which surrounds the baby throughout the pregnancy. Around the cluster of cells there is a single layer of cells which will eventually become the placenta and the cord.

The lining of the uterus thickens until it becomes around four times its usual thickness. By 4 weeks the embryo will develop tiny little roots into the uterus, just like a tree. These connect to the mother’s blood supply and helps the embryo to get all the nutrients and oxygen it needs to develop.

Weeks 5-6

This is the time that the nervous system starts to develop. The neural tube develops which will become the baby’s brain and spinal cord. The heart begins to form and the baby has its own blood vessels. The blood vessels that connect the embryo to the uterus become the umbilical cord.

Weeks 7-8

The major organs and body systems are formed. A heart beat can be seen on an ultrasound. The first bone cells have developed. Bumps form on the body which will become the baby’s muscles and bones. More bumps form on the body which show where the arms and legs are developing. Dimples on the side of the head will later become the ears and there are thickenings where the eyes will be. The embryo is now about 8mm long.

Weeks 9-12

By the end of the third month, the baby is fully formed. It has arms, hands, fingers, feet and toes. It can open and close its fists and mouth. Fingernails and toenails are beginning to develop and the external ears are completely formed. The teeth are starting to form. The reproductive organs develop and the heart is working. The kidneys are working and putting urine into the bladder. The liver is producing bile.

At the end of week 12, the baby is about 1cm long.

What’s happening to Mum?

Weeks 0-12

You’ll be producing heaps of oestrogen and progesterone and the lining of the uterus will thicken very quickly.

Your breasts will probably feel uncomfortable in some way or another. Many women feel fullness and tenderness and you may notice biggish veins starting to develop.

You will probably be spending a lot of time on the toilet as the bladder is affected by the pregnancy hormones and the expanding size of the uterus. You may experience morning sickness and feel really tired .

Towards the end of the first trimester you may notice that your waistline has disappeared and you may start to show.

Most women can’t feel the movements of the baby at this point.

You will be producing lots of different pregnancy hormones and many women find the first trimester to be a bit of a rollercoaster. Rapid moods swings and irritability are common. You may find that opposing emotions often co-exist in a single moment. Blame the hormones. It’s a good excuse, and you don’t get to use it too many times in your life.