Week by Week Development of the Foetus

Week by Week Development of the Foetus

Sometimes ultrasounds give you too little information in visualizing your baby’s week by week growth in your early pregnancy weeks.

How your baby grows and changes it’s shape every week is both interesting and emotional for the a mother to know. Below we’ve explained the week by week development of the foetus:

First Weeks

Week 1 – When it all begins


Day 1 begins with fertilization. Fertilization is the fusion of male sperm and female ovum. The fusion results in the creation of the very first cell of the baby called the zygote.

Division of the Baby’s First Cell

The single-celled zygote now undergoes cell division splitting itself into two cells, then four cells and then into eight cells. The multiple celled zygote is called morula.

Fluid then starts filling in the morula. As fluid fills in the morula, the cells naturally divide into two halves:

  1. Embryoblast (Grows to become the Embryo)
  2. Trophoblast (Grows to become the Placenta)


By the end of the first week of pregnancy, the baby’s cell now gets attached to the mother’s body. This process of attachment is called implantation.

Week 2 – The baby cells further divide

Placental circulation starts off in this week with complete penetration of placental tissue into the uterus.

The formation of the initial layers of the baby now starts. These layers are namely:

  1. Epiblast (forms the future skin and hair of the baby)
  2. Hypoblast (forms the future muscles of the baby)
  3. Prechordal Plate (forms the future mouth of the baby)

Week 3 to 8 – Budding into a Human

Organ formation of the baby starts in week 3.

The baby is medically referred to as the Embryo from this period up till week of pregnancy. The expected length of the baby is about 2-3 mm at the start of week 3 of pregnancy and is expected to measure up to 30 mm by the end of week 8 of pregnancy.

The three major layers of the baby namely the Epiblast, Hypoblast and Prechordal Plate further develop during these weeks.

All organs of the baby start developing during these weeks including the skin, eyes, ears, heart, brain and stomach etc. Especially the baby’s upper body (arms, fingers and elbows etc.) begin to develop and start giving a clearer shape to the baby’s body.

However, no sex determination is possible as genitalia begin to differentiate up till week 8 of pregnancy.

Second Weeks

Week 9 to 12 – Embryo turns into Feotus

Embryo is now called the fetus since it starts forming prominent human bodily features. The expected normal length of the baby is about 45-50 mm in week 9 of pregnancy.

The baby’s brain is developed during these weeks. Toes and fingers of the baby lose its webbed shape and start separating.

Facial features of the baby start to grow and genitals are fully formed by week 10 of pregnancy.

By the end of week 12 of pregnancy, a normal baby is expected to grow in length to about 70-90 mm.

Week 13 to 20 – Your baby kicks

All major organs grow as the baby grows. Formation of the baby’s blood cells also takes place during these weeks.

During these weeks, the baby’s hair starts to grow all over the body and head of the baby. It is during this period that the mother starts to feel baby movements (also known as baby “kicks”).

Sex determination is possible by week 12 of pregnancy.

A normal baby is expected to measure about 135-140 mm by week 13 of pregnancy.

third weeks

Week 21 to 28 – Viable

The baby is viable from the beginning of week 21 of pregnancy. Being viable in pregnancy terms means that the baby can be born with reasonable chances of survival.

Your baby is expected to measure about 220 mm at the start of week 21 of pregnancy and is expected to reach a length of up to 280 mm by week 28 of pregnancy.

The baby now moves its fingers, lips and feet etc. and can open and shut its eyes. Your baby also has its own breathing lungs by this time. Baby sucking on its thumbs can be viewed in 3D ultrasound.

Week 29 – 36 – It’s about time!

The baby is normally expected to measure about 290 mm at the start of week 29 of pregnancy and should normally have a length of up to 350 mm at the end of week 36 of pregnancy (i.e. the ninth month of pregnancy).

During this period, the baby’s bodily hair lessens. However, the baby’s head hair remains thick and dark. The baby can move its hands, feet and head, up and down.

A pregnancy is expected to deliver a healthy baby by the end of the 9th month of pregnancy.

Note: This presentation is a general outlook of embryo inside uterus, to make you understand the growth and shape of the baby inside you. It may vary a little bit with every other pregnant woman depending upon their case as everyone’s case is different.