Medically reviewed by Joseph Aquilina, Consultant obstetrician and gynaecologist
Written by
Lynda Cranston | en français
Your baby measures about the size of a sesame seed (NHS 2018), and looks more like a tiny tadpole than a human (Moore et al 2019a). You won't have a bump just yet, but big changes are occurring inside your uterus.
The developing placenta continues to burrow into the wall of your uterus to ensure a good blood supply for your baby (Hill 2019a, Moore et al 2019b), who is growing at a furious pace (Moore et al 2019b, Visembryo nd a, b, c).
Advertisem*nt | page continues below
Within your baby's tadpole-shaped form, three layers of cells are forming. These will grow into the major organs and tissues (Hill 2019a, Moore et al 2019b, Visembryo nda):
- In the top layer, a hollow structure called the neural tube begins to form, as a groove then a tube. This is where your baby’s brain, backbone, spinal cord and nerves will develop. Skin, hair and nails will also develop from this layer (Moore et al 2019c).
- The middle layer is where the skeleton and muscles grow from, and where the heart and blood circulation system will form.
- The third layer houses the beginnings of what will become the lungs, the intestines and the urinary system.
(Bailey 2014, Moore et al 2019b)
Your baby’s neural tube also starts to close at each end to form a protective cavity around the emerging spinal cord and brain (Hill 2019b, Moore et al 2019b, Visembryo nd d). If you haven't started taking folic acid, do so now. Taking folic acid during the first 12 weeks, will help prevent a neural tube defect from forming at this crucial stage (NHS 2018).
Your baby’s tiny heart is the first organ to function (Hill 2019c, Moore et al 2019b, d). Although it looks more like a tube, rather than the final four chambers of a fully grown heart, it's already connected to blood vessels (Moore et al 2019b, NHS 2018, Visembryo nd c). It’s probably already beating, as most embryo hearts start to function around day 21 or 22 of pregnancy (Hill 2019c, Moore et al 2019d, Visembryo nd d).
Check out our pregnancy guide to see what changes could be happening to you at five weeks pregnant.
VIDEO
Note: every baby develops differently. This page is designed to give you a general idea of how an embryo grows in the uterus.
<< 4 weeks
6 weeks >>
Advertisem*nt | page continues below
Was this article helpful?
- 20 weeks pregnant: fetal developmentBy Lynda Cranston
- 35 weeks pregnant: fetal developmentBy Lynda Cranston
- 24 weeks pregnant: fetal developmentBy Lynda Cranston
- 32 weeks pregnant: fetal developmentBy Lynda Cranston
Track your pregnancy on our free #1 pregnancy & baby app
Sources
BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies.
Bailey J. 2014. Hormonal cycles: fertilization and early development. In: Marshal J and Raynor M. ed. Myles textbook for midwives. 16th ed. Edinburgh: Churchill Livingstone, 91-100
Hill MA. 2020a. Embryology: week 3. University of New South Wales Embryology. embryology.med.unsw.edu.auOpens a new window [Accessed April 2020]
Hill MA. 2020b. Embryology: Neural system development. University of New South Wales Embryology. embryology.med.unsw.edu.auOpens a new window [Accessed April 2020]
Hill MA. 2020c. Embryology: cardiovascular system - heart development. University of New South Wales Embryology. embryology.med.unsw.edu.auOpens a new window [Accessed April 2020]
Moore KL, Persaud TVN, Torchia MG. 2019a. Fourth to eighth weeks of human development. Chapter 5 in The developing human: clinically oriented embryology. 11th ed. Elsevier
Moore KL, Persaud TVN, Torchia MG. 2019b. Third week of human development. Chapter 4 in The developing human: clinically oriented embryology. 11th ed. Elsevier
Moore KL, Persaud TVN, Torchia MG. 2019c. Integumentary system. Chapter 9 in The developing human: clinically oriented embryology. 11th ed. Elsevier
Moore KL, Persaud TVN, Torchia MG. 2019d. Cardiovascular system. Chapter 13 in The developing human: clinically oriented embryology. 11th ed. Elsevier
NHS. 2018. Your baby at 5 weeks. NHS, Health A-Z, pregnancy and baby. www.nhs.ukOpens a new window [Accessed April 2020]
Visembryo. nd a. Carnegie stage 7: neurulation. The Visible Embryo. www.visembryo.comOpens a new window. [Accessed April 2020]
Visembryo. nd b. Carnegie stage 8: primitive pit, notochordal canal and neurenteric canals. The Visible Embryo. www.visembryo.comOpens a new window. [Accessed April 2020]
Visembryo. nd c. Carnegie stage 9: appearance of somites. The Visible Embryo. www.visembryo.comOpens a new window. [Accessed April 2020]
Visembryo. nd d. Carnegie stage10: neural folds. Heart folds begin to fuse. The Visible Embryo. www.visembryo.comOpens a new window. [Accessed April 2020]
Lynda Cranston
Lynda has been a health and medical writer for 20-plus years. She has extensive experience in re-framing and streamlining complex medical information so it is easily digestible and actionable for family physicians, patients, and consumers.
Opens a new windowOpens a new windowOpens a new window
