Baby Development In First Trimester | Weeks 1-13

The time right after finding out that you’re pregnant is one of celebration and happiness. A lot happens in the first three months of pregnancy. Once you start experiencing early pregnancy symptoms, there are a lot of changes during pregnancy that your body goes through. With rapid fetal development during pregnancy, moms-to-be usually want to know the details of their baby development by weeks. This focus on every little thing happening inside your belly is a very natural response to pregnancy. After all, you are nurturing and growing a little human in your womb. 

Technically, the first trimester starts after the start of your last period and ends at approximately week 12. But most of you don’t realise you’re pregnant until you take a pregnancy test in maybe the 5th or 6th week. This trimester, your baby’s development is faster than the other two trimesters. They go from being a single cell to a developed tiny person in the span of these three months. It will always be an exciting experience to actually witness your baby’s growth at each ultrasound appointment. To make these moments even more joyful and memorable, get to know more about fetal development during pregnancy, especially in the first three months of pregnancy.

To get you up to speed on what is happening to your baby in this trimester, we have divided the baby development by weeks into 3 large categories.

Baby Development By Weeks 1 – 4

  • These are the starting weeks of pregnancy. More often than not, they pass by without you even noticing that something is changing in your body because there are barely any visible physical changes too.
  • Since the exact moment or date of conception is hard to guess, the trimester is counted from the start of your last period. The fertilized egg transforms into an embryo after rapid multiplication of cells. This embryo is then implanted into the wall of your uterus. 
  • In this four week period, the embryo could have been implanted on any day and it would still be counted as the week 4 of pregnancy because the counting starts from your last period rather than the actual day of conception.
  • The baby is the size of a poppy seed at 4 weeks and the many layers of multiplying cells will grow into its organs later on. It is unbelievable that your baby has already developed functioning blood vessels. Some of them will later develop into the umbilical cord too.
Baby development size in 4 week

Baby Development By Weeks 5 – 8

  • At 5 weeks, fetal development during pregnancy leads them to be the size of 9mm or almost equal to a fingernail. There is already budding limbs, bone structure and a primitive brain developing in their body.
  • Babies are commonly compared to a tadpole at 6 weeks as they are of similar size and have their backs curved. Your baby’s heart starts to beat around this time as well. The areas where your baby’s ears and eyes will grow will look different from the rest of their head.
  • Looking at the ultrasound image of your baby might be strange at this time because their head is growing faster than the rest of their body, leading to an alien-like appearance. Brain, spinal cord and nervous system development is happening now.
  • At the end of 8 weeks, your baby will finally start to look like a tiny human. Their arms and legs are no longer just buds waiting to grow, but actual thin limbs. With their body uncurling a bit, your baby is now a foetus rather than an embryo whose size has almost doubled.
Baby development size in 8 week

Baby Development By Weeks 9 – 12

  • Your baby is almost the size of a strawberry at 9 weeks into the first three months of pregnancy. Their eyelids will be developed and their eyes will start to get colour in them. Other sensory organs like taste buds will also develop at this stage.
  • At 10 weeks, your baby is around 3 cm long with facial features like lips, nostrils and jaw bone showing. Their genitals are also growing and will later indicate the gender of your child. Another exciting part is the movement they are starting now.
  • Your baby already has functioning kidneys and stomach at 11 weeks. Their fingernails are growing along with the separation of tiny fingers and toes. Their earlier fluttering eyelids will now close until week 26. 
  • About the size of a plum at 12 weeks, the end of the first three months of pregnancy is marked by everything being in place. Your baby is fully formed with limbs, organs, muscles and bones developing. Now all that is left to do is grow. You can’t feel it yet but your baby is moving so much, that he or she might even be dancing!
Baby development size in 12 week

Fetal development during pregnancy is a rapid process, so even if you are not showing yet, it will only take a couple of weeks for you to notice a change in your tummy size. As your baby continues to grow, you will also experience changes during pregnancy including skin changes. Your bump will get bigger as the baby development in the second trimester continues. To cope and adjust more easily with these changes during pregnancy, you can also look into some pregnancy hacks.

Keep track of your baby’s growth and maintain your health in this sensitive period. A lot of this growth is dependent upon what you eat as well. Eat the right foods in pregnancy to not only aid baby development, but to also preserve your own well being. And as the first trimester ends, you will want to know what happens in the next trimester. So read more about week by week baby development in second trimester to stay informed about it.
With regular check-ups from your doctor, make wise choices that are best for both you and your baby. The first trimester checklist of things to do and prepare for later stages in pregnancy might be helpful too. The first trimester will go by the fastest, so make the most of every moment and enjoy the simpler times of your pregnancy!

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