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Fetus Development: How Your Baby Grows Inside the Womb

Medically reviewed by Ann Guevarra MD, OB-GYN Diplomate, POGS · Obstetrics and Gynecology

Written by Jan Alwyn Batara · Updated Jul 18, 2022

    Fetus Development: How Your Baby Grows Inside the Womb

    Fetus development is an amazing thing. In the span of just 9 months, a small bunch of cells turns into your wonderful bundle of joy. But how exactly does your baby grow inside the womb?

    Fetus Development in the Womb

    Before we talk about what happens to your baby’s development during each trimester, we first need to talk about how a fetus is formed in the first place1.

    As everyone knows, it all starts when a sperm cell fertilizes the egg cell inside the woman’s fallopian tubes. Once the egg is fertilized, it quickly starts to multiply into new cells. Eventually, it finds its way to the uterus where it implants itself. Eventually, these cells will form the embryo, which develops into a fetus during the 11th week of pregnancy.

    Implantation is technically when the pregnancy starts, as when it happens, that’s when the body starts to release pregnancy hormones. These hormones act as signals for the body to undergo changes in order for it to be able to care for the baby and carry it for 9 months until birth.

    First Trimester

    The first trimester refers to the first three months of the baby inside the womb. As the embryo starts to develop, the placenta also starts to form during this stage. The placenta is an amazing organ that provides all the nutrition that your baby requires. Aside from this, it also filters out any substances that might cause harm to your baby2.

    One of the first things to develop during the first trimester is your little one’s nervous system. This includes their brain, spinal cord, as well as the nerves. Next is the heart, this will eventually start beating at around 5-6 weeks of pregnancy.

    Next, your baby’s face, arms and legs, sexual organs, and then their muscles will start to develop. At the end of the first trimester, your baby is just about 3 inches long. They still have a long way to go!

    Second Trimester

    By the second trimester, most of the major organs have already started to develop. This means that during this time, most of the fetus development that happens has to do with growth of these organs, as well as your baby3.

    This is also the time when your baby will start to kick, as well as move around in your tummy. Their eyes start moving from the sides of their head to the front, and your baby’s ears will also start to function during the second trimester. It’s a good idea to constantly talk to your baby, as being familiar with your voice can soothe them once they come out of your womb.

    The brain is also undergoing numerous changes during this time, and their reflexes will also start to develop. Fetus development also happens rapidly during the second trimester, and you’ll be surprised at the pace your little one is going.

    Third Trimester

    During the last trimester of fetus development, what’s mostly happening is your little one trying to grow so that they’ll be healthy once you give birth. At this point, their lungs aren’t fully mature, but they’re almost there.

    Your baby will also start to position its head down, in order to get ready for birth. Though, some babies can be stuck in the breech position, which might require physically turning the fetus, or a C-section in order to have a safe birth.

    At this point, all you’ll need to do is make sure that you’re healthy and ready to give birth. Your baby is almost fully grown at this time. Most babies born term (37th to 40th week) are immediately roomed in or immediately taken to their mother. Premature babies (before 37th week) and those born under severe circumstances will stay in the Neonatal Intensive Care Unit.

    Key Takeaways

    Fetus development in humans happens in just 9 months. During this time, your baby goes through a number of significant changes, and its surprising to see how fast a baby develops after such a short period of time.


    Hello Health Group does not provide medical advice, diagnosis or treatment.

    Medically reviewed by

    Ann Guevarra MD, OB-GYN Diplomate, POGS

    Obstetrics and Gynecology

    Written by Jan Alwyn Batara · Updated Jul 18, 2022

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