Stages Of Fetal Development
The First Trimester

To think that your baby starts out as a single cell, and then multiplies over time, and in 40 weeks is a fully developed baby - Wow!

Here We'll look at fetus development, and note the changes as they happen weekly.

While you can't feel a thing that's happening in your uterus, you won't believe the amount of action that's taking place... A true miracle in the making!

Stages of fetal development
Weeks 1 to 4

• When your egg is fertilized, your baby's gender is determined.

• Week 2 (which is 4 weeks after your last period) is the week after fertilization. Your baby at this stage is classed as an embryo. It is about 1/100 of an inch long, and it will carry on growing.

• The embryo's neural tube forms. This will develop into your baby's nervous system.

• Heart and circulatory system form quite quickly.

Stages of fetal development - weeks 5 to 8

Week 5

• The umbilical cord now starts to form, this will be your baby's lifeline until you meet her face to face.

• Your baby's arms and legs start to appear. Known as 'limb buds' these will start to grow into recognizable limbs soon.

• Other organs now start to develop too. The brain and lungs are starting to form.

Week 6

• Your baby is now about 5mm long, and looks a little like a tiny seahorse.

• Your baby's mouth is starting to form, with dental buds inside too!

• Your baby's heart is now beating! and can be clearly heard by ultrasound.

• Blood circulates to and from the placenta.

• The digestive system is forming, with a liver and kidneys. The nervous system is now starting to develop too.

• The intestines are starting to form in loops.

• Your baby has now grown to around 1.3 cm long!

• She now has lips, a tongue, and her nostrils will be starting to form.

• Remember those limb buds? They are now properly formed arms and legs with tiny little hands and feet attached. Fingers and toes will now start to grow.

• Baby's heart now has 4 chambers and is busy pumping blood.

• Your baby is now about 2.5cm long, and starting to resemble a real baby (not a peanut!)

• The face is now recognisable with ears and eyes forming, including the beginning of eyelids.

• Your baby's main internal organs are all inside her tiny torso, with her arms and legs getting longer every day.

Stages of fetal development - week 9 to 13

• Your baby is now starting to move around in your uterus, making use of her developing bones and muscles.

• Baby's finger and toes are growing along with her arms and legs. She know has ankles and wrists developing to go with the set!

week 10

• Baby is now around 4.5cm long

• Ankles and wrist bones have formed now.

• If you were to have an ultrasound at this stage then you would see your baby's fingers and toes are now quite visible.

• Your placenta now starts to take control over your pregnancy.

Week 11

• Your baby's external genitals are now forming.

• Your baby may now make vigorous movements, but she is still too small for you to feel them.

• She is now practising the sucking reflex, this will get her ready for breastfeeding. On ultrasound you may see her searching by turning her head, looking for an imaginary nipple.

• Your baby will now use the the muscles that are for swallowing and breathing when she is born. Practice makes perfect!

• Baby's head is still large compared to the rest of her body, but her neck muscles and bones are now developing and she will start practising lifting her head off her chest.

• By the end of this week, all of your baby's internal organs are fully formed inside her body. For the rest of the pregnancy they will continue to grow.

Weel 12

• Baby's eyelids are formed although closed.

• She will now respond to sounds outside of the uterus.

• Your baby may suffer with occasional hiccups! (how starnge is that!)

• Baby's liver and spleen are now developing, along with blood cells in the bone marrow.

• Baby now has all of her teeth buds in place in her mouth. Ready for when she gets older and has a full set of 'munchers'!

Week 13

• The fetus is really starting to look like a baby now! His limbs and head are starting to look more in proportion with his body.

• You baby now has a soft downy layer of hair over his body. This is called 'lanugo'

• Your baby now has just as many nerve endings as an adult, and important connections between his brain, muscles and nervous system are all being made.

• By the end of this week, your baby's hands and feet are fully formed, with tiny little toes and fingers.

Well that's it for fetus development for the first trimester. It's hard to believe how much has happened over the last 12 weeks. The stages of fetal development are in no way over yet though, so...

If you fancy a peek at the week by week fetal development for the second trimester, then take a look here.

Return from stages of fetal development - first trimester, to the pregnancy page.

Or back to the New baby and beyond homepage.