How your baby is developing - fetus at 3 to 5 weeks

The following articles describe how your baby develops from the day you conceive until you give birth.

Measuring pregnancy weeks  

Doctors and midwives in the UK measure the duration of pregnancy from the first day of your last menstrual period, not from the day you conceive. So when you are ‘four weeks pregnant’, it is actually about two weeks after you conceived. Pregnancy normally lasts for 37–42 weeks from the first day of your last period. The average is 40 weeks.

If you are not sure about the date of your last period, then your early scan will give a good indication of when your baby will be due.

In the very early weeks, the developing baby is called an embryo. From about eight weeks, it is called a fetus.

Embryo at 3 weeks       

This is three weeks from the first day of your last period. The fertilized egg moves slowly along your fallopian tube towards your uterus. It begins as one single cell, which divides again and again.

By the time the fertilized egg reaches your uterus, it has become a mass of over 100 cells, called an embryo. It is still growing. Once in your uterus, the embryo attaches itself into your uterus lining. This is called implantation.

Embryo at 4 to 5 weeks

The actual size of the embryo is about 5mm .The embryo now settles into your uterus lining. The outer cells reach out like roots to link with your blood supply. The inner cells form two – and then later three – layers. Each of these layers will grow to be different parts of your baby’s body. One layer becomes their brain and nervous system, skin, eyes and ears. Another layer becomes their lungs, stomach and gut. The third layer becomes their heart, blood, muscles and bones.

The fifth week is when you will miss your period. At this time, most women are only just beginning to think they may be pregnant.

Already your baby’s nervous system is starting to develop. A groove forms in the top layer of cells. The cells fold up and round to make a hollow tube called the neural tube. This will become your baby’s brain and spinal cord, so the tube has a ‘head end’ and a ‘tail end’. Defects in this tube are the cause of spina bifida. The heart is also forming and your baby already has some blood vessels. A string of these blood vessels connects your baby to you – this will become the umbilical cord.