Pregnancy Week 4 – The Baby’s Progress

Pregnancy Week 4 continues with the developing heart and nervous system. The hollow cardiac tube will now form a septa (a wall dividing a cavity or structure) and develop into 2 channels, the aortic and pulmonary. From there it will take on an asymmetrical S-shape (as shown below). Endocardial muscle (the innermost layer of tissue that lines the chambers of the heart) will begin to fuse together and once this happens, valves will appear and cardiac contractions will begin. This rhythmic muscle contraction will propel fluids throughout the body to begin with, although this is not yet true circulation because blood vessel development is still incomplete.

Pregnancy Week 4 What To Expect – Fetal Heart Development

However, the primary blood vessels along the central nervous system are now beginning to connect so this is, so far, the most developed system in the embryo, although, at this stage, the brain and spinal cord together are the largest and most compact tissues of the embryo.

As the somites (see week 3) continue to develop along the neural tube it will begin to close and once this happens the cells that become the eyes will appear as thickened circles just off the neural folds. Once 20 somites are in place the forebrain will close. The newly differentiated cells of the ears are also present and the arches that form the face and neck are beginning to appear under the brain. The beginning cells of the liver will form before the rest of the digestive system.

During pregnancy week four your baby is fast developing into a discernible looking shape away from the hollow cluster of cells it was just a couple of weeks ago. The water bubble that the embryo is still floating in will soon develop into the amniotic sac which will continue to house and protect the baby throughout term. The embryo is now connected to the developing placenta via a small stalk that will later become the umbilical cord.

Pregnancy Week Four

This week your embryo has a bulb tail and will curve into a C-shape with upper limb buds appearing. The embryo is measured ‘crown to rump’ which means ‘head to bottom’. The tail is not included as it will disappear by the end of the ’embryonic stage’. Once the limbs have fully developed by the second trimester your baby can either be measured ‘crown to rump’ or ‘crown to heel’ (head to toe).

During these early weeks of pregnancy the embryo growth rate is so precise that your pregnancy can be dated by measuring the exact measurements of the embryo. It is only once you reach the 2nd trimester that differences in growth rates will begin to appear.

The embryo growth in pregnancy week 4 is now around 4 mm crown to rump and is a perfect C-shape.

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(See 4 Weeks Pregnant– The Mother’s Progress)


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