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Module 02

The big picture: from two cells to a newborn

The whole of human development in one module: how egg and sperm become an embryo, a fetus and a baby, and how to read the two different ways of counting weeks.

  • 5lessons
  • 20 minreading
  • 6quiz questions
Start the module Take the quiz

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 02

  1. Lesson 2.1 · 4 min The whole story on one page A map of human development from egg and sperm to birth, so every later module has a place to sit.
  2. Lesson 2.2 · 4 min Two clocks for counting weeks Why "6 weeks pregnant" and "a 4-week-old embryo" describe the same moment, and which clock this course uses.
  3. Lesson 2.3 · 4 min Weeks 1 and 2: from fertilisation to implantation How one cell becomes a hollow ball of about a hundred cells and then settles into the lining of the uterus.
  4. Lesson 2.4 · 4 min Weeks 3 to 8: building the body plan How a flat disc of cells becomes an embryo with a beating heart, a brain and limbs in six weeks.
  5. Lesson 2.5 · 4 min Week 9 to birth: growing and getting ready What changes in the long fetal period, why the lungs come last, and how much the length of a healthy pregnancy varies.
  6. Quiz6 questions · 5 to passNot taken BadgeModule 02Earned with the quiz

Learning aims

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

  • Describe the main stages of development in order, from eggs and sperm to birth.
  • Explain the difference between weeks after fertilisation and weeks of pregnancy, and convert one into the other.
  • Recognise what happens in the first two weeks, including when implantation usually begins.
  • Explain what the embryonic period is, and name the three germ layers and what they form.
  • Compare the embryonic period with the fetal period, and explain why the length of a healthy pregnancy varies.

The module in brief

The module in brief
  • Development runs from eggs and sperm, made by meiosis, through fertilisation, the first week, implantation, the embryonic period and the fetal period to birth.
  • Embryology counts weeks after fertilisation; pregnancy care counts from the first day of the last period, which runs about two weeks ahead.
  • By about day 5 after fertilisation the embryo is a blastocyst; implantation usually begins around day 6 to 7, and hCG is first detectable 6 to 12 days after ovulation, most often on day 8 to 10.
  • In week 3 after fertilisation the embryo forms three germ layers: ectoderm, mesoderm and endoderm.
  • The embryonic period is the first 8 weeks after fertilisation, when every major organ system begins; the heart starts to beat at around day 22.
  • From week 9 the developing baby is a fetus, and the rest of pregnancy is mostly growth and maturing, with the lungs among the last organs to be ready.
  • Birth comes at about 38 weeks after fertilisation (40 weeks of pregnancy), and healthy pregnancies vary in length by several weeks.
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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