By Cristina Mestre Ferrer BSc, MSc (embryologist).
Last Update: 07/16/2014

During the fourth month of pregnancy nauseas are no longer a nuisance, as well as other typical of the first semester annoyances. The hormonal levels are stabilised and now the body is ready to deal with all the changes that are about to happen.

The different sections of this article have been assembled into the following table of contents.


During this month the embryo undergoes some important changes:

  • The size increases from 8.5 cm at the beginning of the fourth month to roughly 15 cm at the end.
  • The weight also increases. The amount may vary but usually ranges from 100gr to 200gr.

Fourth month

At the fourth month the eyes are huge, and remain closed and quite separated from each other. The foetus is covered by a fine layer of hair called lanugo.

During these weeks you will notice its movement for the first time. At first you will notice a smooth waving, but soon you will unmistakably feel the movement of the living being that is growing inside you.

The vocal cords are also formed to be used after birth. The future baby grows from
8.5 cm of length at the beginning of the month until approximately 15 cm at the end of the fourth month, then having an average weight of 170 gr.

The fourth month goes from week 13 to week 16. Let’s take a look on each week.

13 weeks pregnant

During the 13th week, a layer of hair starts to grow all over the foetus’ body, nearly covering it. This thin hair is called lanugo.

The foetus has now a grown neck and its face is well-formed, although the chin is still small and the mouth big if compared to the tip of the chin. The eruption of teeth is starting to form.

Prengnacy at the 13th week

14 weeks pregnant

At the 14th week, a small layer of hair grows in the head and the eyebrows are formed.

The external ear moves upwards, next to its definitive location in the head. The eyes still remain huge, closed and separated.

The sweat and sebaceous glands of the baby-to-be start to be active. The intestine begins to be filled with a thick substance secreted by the liver and called meconium.

15 weeks pregnant

At the 15th week, the hair of the head grows and the eyebrows thicken.

If the future baby has the genes for dark hair, the pigmentary cells of the hair follicles start to develop the black pigment.

The muscles of the face already make the sucking motion and the foetus now is able to suck its thumb. Thanks to this, it prepares its muscles for breastfeeding once is born.

16 weeks pregnant

In week 16, the baby-to-be keeps growing and the head and the body are more proportionated. The neck is starting to have its proper form.

The epidermal fold of the palm of the hand and fingers appear. Now the new being has already fingerprints. The hematopoiesis of the bone marrow is underway. The hematopoiesis is the formation of blood cells.

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Authors and contributors

 Cristina Mestre Ferrer
BSc, MSc
Bachelor's Degree in Biological Sciences, Genetics & Human Reproduction from the University of Valencia (UV). Master's Degree in Biotechnology of Human Assisted Reproduction from the UV and the Valencian Infertility Institute (IVI). Embryologist at IVI Barcelona. More information