Fetal Development in the First Trimester: Stages & Timeline

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  • 0:08 Embryogenesis
  • 3:30 Organogenesis
  • 4:41 1st Trimester: Weeks 0-4
  • 5:15 1st Trimester: Weeks 5-8
  • 6:33 1st Trimester: Weeks 9-12
  • 7:59 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

We all know it takes 9 months for the baby to develop inside the mother's womb, but have you ever thought about how or when all those different organs and systems appear? When does the fetal heart start beating or the brain develop? Find out in this lesson covering the first trimester of pre-natal development.


Amazing, isn't it? - that we all started out as a tiny, microscopic ball of cells that just kept dividing and dividing! Then over time that tiny little ball of cells organized itself into different layers called germ layers. You may remember from previous lessons that you have three of these layers:

  • an inner endoderm
  • a middle mesoderm
  • an outer ectoderm

The cells that make up our body are organized into three germ layers
Germ Layers

Did you know that all of the organs and tissues in your body come from one of these three layers? See, amazing, right? That all of your organs, whether as simple as a single cell, or as complex as your brain - all of them come from this tiny microscopic ball of cells!

The outer ectoderm cells become structures like your outer skin layers and your inner nervous system cells.

The middle mesoderm cells form many of the structures that lie in between your outer skin and your inner body cavities. These mostly include your muscle cells, such as skeletal, smooth and cardiac muscle, but it also includes other cells like the red blood cells that run through your arteries and veins and the cells in your kidneys.

And those inner body cavities? Well, they are filled with your organs, and organs come from the inner endoderm. These include your lungs and respiratory system, your liver, your digestive system and intestinal tract and your endocrine glands.

Cell Differentiation

So, how does it happen? Well, throughout development your cells receive signals that tell them what to do. These signals come from the genes (you know, the ones in your DNA) that are inside each cell. A cell's genes are kind of like its instruction booklet - they tell the cell where to go and what to do. All of your cells have their own chosen destiny, kind of like a chosen career path. Some are chosen to become heart cells, others brain cells or muscle cells and so on. This process is called differentiation.

During cell differentiation, cells are chosen to become specific types of tissues
Cell Differentiation

Differentiation is the process during which a non-specialized cell becomes specialized. You can think of it like going to college without a clue of what you want to do in life and coming out with a degree in biology, business, art or history. You went in unsure of your path and came out with a specialized degree that will shape the rest of your career. That's kind of like how cell differentiation works.

Each cell starts out the same; they all have the potential to turn into whatever they want, but the body needs cells of all different types. So, just like your education refines you and guides you along your career path, a cell's genetic instructions refine it and guide it along its path. Once differentiated, a cell cannot go back. A heart cell can't decide one day to be a brain cell, just like someone trained in architecture can't wake up one day and decide that they're a surgeon. An architect lacks the necessary training, just like the heart cell lacks the necessary structure and shape to function in the brain.


Okay, so once each cell has gotten its instructions, then what? Well, following the process of cell differentiation comes another process called organogenesis. Organogenesis is the process by which differentiated cells organize themselves into tissues and organs. This starts as early as 5 weeks after conception in humans, when the developing embryo is no larger than a sesame seed!

Don't worry - we aren't going to go into too many details. After all, this can be a very complex topic. I mean, organ development! And you thought just learning how they work was hard enough! Instead, let's take a look at the developmental timeline, when each of the major organ systems in your body develops over those 9 months you sat in your mother's womb.

We all know that gestation takes about 9 months. These 9 months are divided into three parts or trimesters. Each part consists of three months, hence the word trimester. So let's start right there, in the first trimester.

1st Trimester: Weeks 0-4

The placenta begins to form at the end of the first month of gestation
Placenta Formation

We'll start with what you probably already know - the first month after fertilization is pretty much just lots and lots of cellular divisions. By the end of the first week, the embryo has made it to the uterus and is in the process of implanting itself into the uterine wall. From there, it enters the next stage of development, the formation of the three germ layers. By the end of the first month, the placenta begins to form and cell differentiation is well under way. If you need a little refresher on this stage, be sure to watch some of the other lessons on our site.

1st Trimester: Weeks 5-8

Now, the second month is where things start to get interesting. Around week 5, our tiny sesame seed-sized embryo starts to grow and change in size and shape.

The limb buds will form the arms and legs
The Limb Buds

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