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Pregnancy Week by Week - 17 to 20 weeks

27 Nov, 2023

A weekly breakdown from 17 to 20 weeks

Week 17

Your baby is the size of an onion. Adipose tissue or fat stores are forming under baby’s skin. This is essential for energy, insulating the body, protecting organs and defining features. The umbilical cord is becoming thicker and stronger to nourish baby. The placenta is expanding and circulation is increasing to deliver nutrients, oxygen and remove waste products. The external sex organs are fully formed to reveal baby’s gender on ultrasound. Loud noises may make baby startle. Eyelashes and eyebrows are growing and baby now has their own unique fingerprint.

Week 18

Your baby is the size of a sweet potato. The nervous system is developing and beginning to form a layer of myelin. Myelin is a substance containing fat and protein that insulates and protects nerve cells including the brain and spinal cord. It helps to send messages along the nerve pathways. More complex connections are made in the brain responsible for baby’s senses including sight, smell, touch, taste and hearing. The ears are in their permanent position and pop out from the side of the head, while the eyes now face forward and can detect light. The bowel is filling with meconium which will be baby’s first poo after birth.

Week 19

Your baby’s sensitive skin is now covered in a waxy white substance called Vernix Caseosa. This important coating creates a waterproof layer to protect the skin from the surrounding amniotic fluid, prevents infection, regulates temperature, promotes wound healing and acts as a lubricant to help your baby pass thorough the birth canal. The lungs are developing and forming the main airways called bronchioles. Your baby has evident waking and sleep cycles, like that of a newborn. Your baby is the size of a mango and beginning to store brown fat deposits beneath the skin to provide warmth once baby leaves the womb.

Week 20

Your baby is now the size of a banana. Permanent teeth have grown below baby’s first teeth, deep in the gums. Your baby is very active as the muscles have matured. Quickening is the term used to describe the first movements or fluttery sensations felt. The development of human egg cells begins before birth and the number of eggs in the ovaries of baby girls have peaked between six and seven million. This number decreases from this point onwards and continues to reduce throughout life. A morphology ultrasound may be conducted to assess your baby’s development.

Written by PBC Expo Midwife Hayley Hall

Midwife, Birth Educator and Mum of 4

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