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Module 08

Weeks 4 to 8: the embryonic period

From week 4 to week 8 after fertilisation (about weeks 6 to 10 of pregnancy), the embryo folds into a body, its heart starts to beat, and its face, limbs and every main organ begin to form.

  • 4lessons
  • 16 minreading
  • 6quiz questions
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The lessons and the quiz in this module are part of the Fertility with Kara Package. Module 1 is free to read.

Your way through Module 08

  1. Lesson 8.1 · 4 min Week 4: from flat disc to curved body How the flat, three-layered disc rolls up into a curved body with a gut tube inside, a heart in front and the beginnings of an umbilical cord.
  2. Lesson 8.2 · 4 min The first heartbeat, and what an early scan shows When the heart starts to beat, how fast it goes, and how to make sense of an early pregnancy scan, including one that is too early.
  3. Lesson 8.3 · 4 min Weeks 5 to 8: face, limbs and the Carnegie stages How the embryo grows from a 5 mm curl to a 3 cm body with fingers, toes and a face, and how embryologists name the stages along the way.
  4. Lesson 8.4 · 4 min Why these weeks are the most sensitive Why the weeks when organs are forming are also the weeks when development is most easily disturbed, and what that means before and in early pregnancy.
  5. Quiz6 questions · 5 to passNot taken BadgeModule 08Earned with the quiz

Learning aims

By the end of this module you will be able to:

  • Describe how the flat disc folds into a curved body in week 4, and what folding achieves.
  • Explain when the heart starts to beat and how much an early pregnancy scan can show.
  • Recognise the main external changes of weeks 5 to 8 and name the Carnegie stages.
  • Explain why weeks 3 to 8 after fertilisation are the most sensitive time for the main organs and body shape.
  • Ask your doctor about medicines, folic acid and rescans in early pregnancy.

The module in brief

The module in brief
  • In week 4 after fertilisation the flat disc folds into a curved body: the heart moves to the chest, part of the yolk sac becomes the gut tube and the umbilical cord begins.
  • Paired somites form along the back from about day 20 and build the vertebrae, ribs and back muscles.
  • The heart tube starts to beat at about day 22 after fertilisation, just over 5 weeks of pregnancy.
  • On a vaginal scan a heartbeat is usually first seen at about 5 and a half to 6 and a half weeks of pregnancy, so an earlier scan without one usually needs a repeat scan before any conclusion is drawn.
  • In weeks 5 to 8 the embryo grows from about 5 mm to about 3 cm and forms its limbs, face and the beginnings of every main organ.
  • Embryologists describe this period as Carnegie stages, defined by features rather than age.
  • Weeks 3 to 8 after fertilisation are the most sensitive time for the main organs and body shape (the brain stays sensitive throughout pregnancy), which is why folic acid, no alcohol, no smoking and a medicines review matter before trying.
A note from Kara

Thank you for choosing to learn with me

I know you had many places to look for answers, and you chose this course. Thank you. It means a lot to me that you trust me with something this close to your heart.

Every lesson here is something I wish every couple could hear before they sit down with their clinic: how a life begins, what happens in the lab, and what the words on your report mean. Take it slowly. Short lessons, one at a time, are the way the science sinks in.

If a lesson raises a question about your own body or your own embryos, write it down and take it to your healthcare professional. That is exactly what this course is for.

Warmly, Kara

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