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Your growing baby: a week-by-week guide to pregnancy

Growing a baby is an amazing process

A glowing pregnant woman in yellow dress smiles to camera while holding her baby bump
A glowing pregnant woman in yellow dress smiles to camera while holding her baby bump

Growing a baby is an amazing process. It’s tempting to focus on what will happen after your little one arrives, but whether this is your first baby or you already have children, it can be a fascinating experience to learn about your baby’s milestone developments.

Let’s break it down into trimesters to help you appreciate the wonder of the life starting inside of you.

The first trimester (Conception to 12 weeks)

Weeks 1 to 3

Your pregnancy is calculated from the first day of your last period. Once your egg is fertilised by a sperm, your body will boost its production of a hormone called progesterone. This tells the endometrium (the innermost lining of the uterus) to prepare for the egg’s implantation.

It can take anywhere from three to 10 days for a fertilised egg to make its way down the fallopian tube to the uterus.

Weeks 4 and 5

By about the end of week 4, you may start to experience some very early pregnancy symptoms, such as sore breasts, fatigue or nausea.

The fertilised egg is called a ‘blastocyst’ and measures about 4-5mm. Inside, the three core layers that will later make up parts of the human body are forming:

  • Ectoderm – nervous system, brain, hair and skin

  • Endoderm – gut and other internal organs

  • Mesoderm – skeleton, blood systems and muscles.

At this time, the placenta and amniotic sac are developed.

Weeks 6 and 7

Guess what? You likely now have two hearts beating inside of you – yours and your baby’s! Your baby’s heart beats very fast – around 150-180 beats per minute.

During these weeks, the bud-like beginnings of their arms and legs start to form, while their jaw and their vertebrae structures also develop. The “stalk” that becomes the umbilical cord – the cord between your baby and the placenta – has appeared and will eventually be their belly button.

Your baby’s head is much bigger than their body, and their ears, eyes, nostrils and lips are starting to form. Meanwhile, their internal organs – stomach, kidneys and lungs – continue to develop.

It’s amazing to think that all this is happening in a baby just 1cm long!

Weeks 8 and 9

It’s likely the changes you’re feeling are starting to make your pregnancy really sink in. You may have gained a little weight and could need to pee more often, because your uterus is growing to make space for your baby, which puts pressure on your bladder.

As for your baby, they’re about the size of a peanut. Their skeleton continues to form, and they’re growing tiny fingers and toes. It’s hard to believe, but your baby already has taste buds!

Weeks 10 and 11

Your baby is now about the size of a prune and will grow another 1cm to reach the size of a fig.

Their organs are formed but aren’t yet working. They also have their internal sex organs (ovaries or testicles). By this time, their brain and nervous system have almost finished developing and their first teeth are already in their gums.

Your baby is making their first movements, although it will be a bit longer until you feel them. The webbing in between their fingers has disappeared and they can almost touch their face.

Week 12

You’ve made it to the last week of your first trimester!

Your baby is about 6cm long, or the size of an apricot, and weighs about 18g.

Although tiny, your baby is now fully formed. All their organs, muscles, limbs and bones are in place. Your baby is busy swallowing amniotic fluid and passing urine through functioning kidneys, and is also practising breathing.

Pregnant woman floats on back in pool with eyes closed

Second trimester (Weeks 13 to 27)

The second trimester is a time when a lot of women get that “pregnancy glow” and start to feel really well.

As for your baby, it’s time for some serious growing. At the start of this trimester, they’ll be about 7.5cm long (about as long as a small orange) and weigh 30g. But by week 27, they’ll weigh around 920g and measure an impressive 24cm from head to toe.

Your baby is already swimming around in the amniotic sac, and by about week 18, you’ll be able to feel this “fluttering” movement. An exciting moment!

Between weeks 18 and 20, you will be offered an ultrasound to check for structural abnormalities, the placenta’s position and if you have multiple pregnancies. Your baby’s external sexual organs will also be visible in this scan.

From week 20, your baby can hear your heartbeat, voice and other muffled sounds from the outside world, and their unique fingerprint is now developed. At week 24, your baby will start opening and closing their eyes and will make a small breathing movements with their lungs.

Third trimester (Weeks 28 to 40)

You and your baby are on the home straight! During this final trimester, your baby will likely grow another 25cm or more to reach about 50cm long. They will continue to put on weight, with the average full-term baby reaching about 3.5kg at birth, and with a head circumference of about 35cm.

At about 32 weeks, your baby is likely to have moved into the head-down position in your uterus. They’ll move even further into your pelvis by week 36, which may cause some discomfort to your bladder.

Your baby will be getting a bit cramped and may not be as active as they were in previous weeks, but you should feel them move throughout every day. If you don’t feel them move, or are worried about their movement, call your doctor or midwife.

At the end of 37 weeks, your baby has reached full term. The huge changes that have been happening in your body, and your large belly, might be taking a toll on your comfort and ability to sleep. Try and get extra rest where you can.

As you get closer to 40 weeks, your baby’s growth slows down. They’re saving energy for their birth – and meeting you for the first time.

nib Nurture

nib has partnered with trusted Australian pregnancy experts Nourish Baby to create a guide that covers everything you need to know so you can enjoy a happy, healthy pregnancy and beyond. 

The nib Nurture program brings you a comprehensive 3-course bundle which will help you develop the knowledge and skills you need to enjoy a healthy pregnancy, positive labour and birth, and feeding success.