A typical pregnancy lasts 40 weeks, but it can be anywhere from 37 to 42 in length. A pregnancy has three trimesters, each lasting for between 12 and 14 weeks. In each trimester, your body will change in different ways and your baby will grow and develop. Knowing what changes are likely to happen to your body can help you plan and prepare.
Pregnancy date counting starts on the first day of your last period and conception takes place in week 2.
First trimester – weeks 1 to 12
The first trimester is the most important time in your baby’s development as it’s when all the major internal and external body structures form. It’s important to avoid smoking and alcohol during this time and to make sure that any medications you’re taking are ok to take during pregnancy. If you have any doubts, it’s best to ask your doctor as these habits can cause serious pregnancy complications and birth defects.
How your body changes
During the first trimester of your pregnancy, you may notice that your breasts are becoming fuller and more sensitive. You may also notice a darkening of the areola (the skin surrounding your nipple). You may start urinating more regularly. This is normal and is a result of your growing uterus putting pressure on your bladder.
You might also notice some of the following changes to your body:
- headaches
- tiredness
- dizzy and light-headedness
- cravings or revulsions for certain foods
- nausea and vomiting
- heartburn
- constipation
Some women find they need to rest more than usual during the first trimester. By the end of the first trimester, some women develop a small bump in their lower tummy.
How your baby develops
Your baby’s development changes week to week during the first trimester. During week 3, the fertilised egg moves along the fallopian tube towards the womb (uterus). They start out as a single cell and divides and multiples into over 100 cells by the time they reach the womb. By week 4, your baby is an embryo and firmly attaches to the inside wall of your womb and starts to grow and develop. At the end of week 5, they are around 2.5mm long and their brain, heart and spinal column will start to form.
By week 6, your baby is starting to look a bit like a tadpole. They’re around 5 mm long by now and their heart will be beating quickly. By the end of week 7, arms and legs, and internal organs are starting to take shape, and by the end of week 8, the face is starting to form. Week 9 will see the embryo at around 1.7 cm long and the facial features are becoming clearer. The skeleton is also starting to form.
By the end of week 10, all your baby’s organ will be formed, but not functioning yet and the skeleton will be continuing to develop. By week 11, the baby is around 4.5 cm long and weighs around 10 grams. Their fingers are also starting to grow nails and the brain and nervous system are almost completely developed.
You can read more about the changes happening in the first trimester at the Raising Children Network.
Second trimester - weeks 13 to 27
Most of the hard work of forming your baby has now been done, and you may start to feel a bit better. During this trimester your baby will increase its size by three to four times, meaning your bump is about to grow!
How your body changes
While the nausea and tiredness has started to lift, you may start to experience some leg cramps and heartburn. Your breasts will continue to grow, and if you haven’t already done so, now is the time to get fitted for a maternity bra. You may experience some milk leakage from your breasts. Some women will notice a faint brown line up the middle of their abdomen (linea nigra) or some pigmentation on their face (chloasma). You might also notice backache, varicose veins or nasal congestion. You may also experience some Braxton Hicks contractions from 26 weeks. These painless contractions prepare your uterus for labour.
How your baby develops
By the end of week 13, your baby is about 7.5 cm and weighs 30 grams. Bones are hardening, and hair is starting to grow on your baby’s head and eyebrows. By week 15, your baby should measure around 9.5 cm and weigh about 80 grams. They may start to suck their thumb. By week 16, your baby’s eyes are still sealed but they’re able to make facial expressions. By week 17, external sex organs have formed and your baby’s kidneys have started working. Your baby may be able to hear loud noises. At week 23, your baby measures around 20 cm and weighs 540 grams and can start to recognise light and pain. By week 27, your baby weighs about 920 grams and can open their eyes.
To find out more about how your baby develops week to week at the Raising Children Network.
Third trimester – weeks 28 to 40
Congratulations, you’ve reached the third trimester and the countdown is on to when you’ll be holding your baby in your arms! This is the trimester when all of your baby’s organs and systems will mature, ready for birth. Your baby is by now getting big and, with less room to move in the tummy, you are likely to feel much more movement as they explore.
How your body changes
The most obvious physical change during the third trimester is your tummy, which by now should look clearly pregnant. You may also experience some swelling and aches and pains as well as backache, heartburn or indigestion.
Carpal tunnel syndrome is also quite common during the third trimester. The carpal tunnel is a small passage in the wrist where nerves and tendons pass from your arm into your hand and fingers. During pregnancy, hormones cause you to retain fluid and they also soften the ligament that forms the roof of the tunnel. When this happens, the nerve running through the tunnel may become squashed. Symptoms are likely to resolve after the birth of your baby.
How your baby develops
During the third trimester, your baby will gain a lot of weight, start practicing breathing and their hair will continue to grow. By week 33, your baby should weigh around 1.9 kg and measure about 29 cm in length and by week 37, they weigh about 3 kg and measures 35 cm. Your baby will be considered “full term” once it reaches 38 weeks, and is likely to be born at any time. If you’re still pregnant at week 40, don’t worry. Very few babies arrive on their due dates and around half of all pregnancies go past 40 weeks. For most women, labour starts naturally at weeks 37 to 42.
To find out more about how your baby develops week to week at the Raising Children Network.
Where to get more information
Phonelines
Online resources
Sources
https://www.pregnancybirthbaby.org.au/braxton-hicks-contractions
https://raisingchildren.net.au/pregnancy/week-by-week
https://www.miraclebabies.org.au/
https://www.healthline.com/health/pregnancy/
https://www.pregnancybirthbaby.org.au/braxton-hicks-contractions
https://www.webmd.com/baby/qa/what-does-frequent-urination-during-your-first-trimester-of-pregnancy-mean
https://birthchoices.raisingchildren.net.au/index.html
https://www.health.gov.au/resources/pregnancy-care-guidelines/part-i-common-conditions-during-pregnancy/carpal-tunnel-syndrome
All information contained in this article is intended for general information purposes only. The information provided should not be relied upon as medical advice and does not supersede or replace a consultation with a suitably qualified health care professional.