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Pregnancy Week by Week - First Trimester

27 Nov, 2023

A weekly breakdown of your first trimester

Week 4

Your baby has implanted into the uterine lining and is currently a collection of cells called a blastocyst. These cells become an embryo this week and form the placenta to nourish and protect baby. Baby is the size of a poppy seed and your child’s characteristics such as sex, eye colour and hair colour have already been determined through its chromosomes.

Week 5

A missed period is experienced. The rise in HCG hormone levels will typically be detected in the urine on a home pregnancy test. Your baby is the size of an apple seed and is positioned inside the amniotic sac, a bag of fluid that provides protection. The heart, brain, spinal cord and blood vessels have started to develop. Early pregnancy signs including tender breasts, nausea and fatigue are common.

Week 6

Your baby is growing rapidly, doubling in size and is comparable to a grain of rice. The jaw, eyes and vertebra are starting to develop and little buds will soon form the arms and legs. A stalk attaches to the front of the baby’s body which will grow into the umbilical cord. Although your baby’s heart is still developing, it is the first organ to function and begins to beat. The heartbeat may even be detected on vaginal ultrasound.

Week 7

Your baby is the size of a blueberry and 10,000 times bigger than at the time of conception. The face is forming the eyes, mouth, nostrils, tongue, eyelids and ears. Internal organ development is occurring including the stomach, kidneys, bowel and lungs. The brain is growing rapidly resulting in the head looking bigger than the rest of the body. The limb buds start to form cartilage which will develop into bones in the arms and legs. The placenta is starting to attach to the womb to provide oxygen and nutrients to baby.

Week 8

Your baby is growing at the rate of a millimetre a day. Hands are evident at the end of the arm buds and webbed fingers are starting to form. The reproductive organs are beginning to develop to soon determine the sex of your baby. Facial features are taking shape and baby is now the size of a raspberry. Body changes and pregnancy discomforts are common including weight gain, bloating, tender fuller breasts and morning sickness.

Week 9

Your baby’s muscles are developing allowing the arms and legs to move spontaneously however it will still be months before these movements are felt by the pregnant parent. The skeleton is forming and the limbs can bend. Tiny blood vessels are visible under the transparent skin and baby is now the size of a grape. Major organ development continues to occur including the pancreas and gallbladder. Eye colour is beginning to appear and taste buds are forming on baby’s tongue.

Week 10

Your baby is the size of a strawberry and has developed internal sex organs, ovaries or testicles but are yet to form externally. The brain is active with brain waves and the heart has four separate chambers. The fingers are no longer webbed and the knees, wrists, ankles and elbows are present. Tiny tooth buds are appearing under the gums and your baby can swallow. The kidneys are producing large quantities of urine and baby boys have already begun producing testosterone.

Week 11

The body of baby is straightening, the torso lengthening and is now the size of a lime. Your baby will start to inhale and exhale small amounts of amniotic fluid which helps the lungs to grow and develop. The muscles and nerves are starting to work together causing baby to have small jerky movements. Hair follicles are forming on baby’s head and the fingers and toes are growing nails. The nostril passages are open and the tongue has formed. The stomach has begun producing gastric juices and your baby has about twenty baby teeth present in the gums.

Week 12

Your baby has formed most bodily systems with limbs, muscles, organs and bones now in place. The placenta is functioning to deliver oxygen and nutrients to baby, removing waste products. Bone marrow has started to produce white blood cells to help fight future infections. The pituitary gland at the base of the brain has begun producing hormones to enable a baby girl to carry babies of her own in many years to come. The kidneys are working to pass urine as baby swallows the amniotic fluid. Baby is now the size of a pear. The chest rises and falls to practice breathing movements.

Disclaimer: The fruits and vegetables used each week is a guide only and aims to compare baby's approximate length and weight. Each pregnancy and baby are unique.

Written by PBC Expo Midwife Hayley Hall

Midwife, Birth Educator and Mum of 4

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