When you are pregnant, you are not only carrying a new life inside of you but you are also responsible for building the baby’s internal organs. It is important to understand when the major organs develop in a baby because it can help you take care of yourself and your baby during your pregnancy. In this article, we will discuss the timeline of when the major organs develop in a baby.
Table of Contents
Week 4-5: Heart, Brain, and Digestive System

During week 4-5 of pregnancy, the baby’s heart, brain, and digestive system begin to form. The heart starts to beat at around week 5, which is an exciting milestone for expecting parents. The neural tube, which develops into the brain and spinal cord, also starts to form during this time. The digestive system is also starting to form, which includes the mouth, esophagus, stomach, and intestines.
Week 6-8: Liver, Kidneys, and Lungs
During week 6-8 of pregnancy, the baby’s liver, kidneys, and lungs begin to form. The liver is responsible for producing bile, which helps break down fats in the digestive system. The kidneys are responsible for filtering waste products from the blood and producing urine. The lungs start to form during this time, which is an important milestone for the baby’s respiratory system.
Week 9-12: Genitalia and Spleen
During week 9-12 of pregnancy, the baby’s genitalia and spleen begin to form. If the baby is a boy, the testes will start to form during week 9. If the baby is a girl, the ovaries will start to form. The spleen is responsible for filtering blood and producing white blood cells.
Week 13-16: Muscles and Bones
During week 13-16 of pregnancy, the baby’s muscles and bones begin to form. The baby’s skeletal system is starting to develop, which includes the bones, cartilage, and joints. The muscles are also starting to develop, which will eventually allow the baby to move.
Week 20-24: Hair and Skin
During week 20-24 of pregnancy, the baby’s hair and skin begin to form. The hair follicles start to develop, which will eventually grow into the baby’s hair. The skin is also starting to form, which includes the epidermis, dermis, and subcutaneous tissue.
Week 28-32: Nervous System and Immune System
During week 28-32 of pregnancy, the baby’s nervous system and immune system begin to form. The nervous system includes the brain, spinal cord, and nerves, which are responsible for controlling the body’s functions. The immune system is starting to develop, which includes the white blood cells and antibodies.
Week 36-40: Final Development
During week 36-40 of pregnancy, the baby’s organs are fully developed and the final stages of development are taking place. The baby is preparing for life outside of the womb and is gaining weight and developing their lungs.
In summary, the timeline of when the major organs develop in a baby is as follows:
- Week 4-5: Heart, Brain, and Digestive System
- Week 6-8: Liver, Kidneys, and Lungs
- Week 9-12: Genitalia and Spleen
- Week 13-16: Muscles and Bones
- Week 20-24: Hair and Skin
- Week 28-32: Nervous System and Immune System
- Week 36-40: Final Development
It is important to note that these are approximate timelines and each baby develops at their own pace. If you have any concerns about your baby’s development, it is always best to consult with your healthcare provider.
Frequently Asked Questions
Q: Can the development of the baby’s organs be affected by the mother’s lifestyle choices?
A: Yes, the development of the baby’s organs can be affected by the mother’s lifestyle choices such as smoking, drinking alcohol, and not getting enough nutrients. It is important for expecting mothers to take care of themselves and avoid harmful substances.
Q: Can a baby survive if born prematurely?
A: It depends on how premature the baby is and what medical interventions are available. Babies born after 24 weeks of gestation have a higher chance of survival, but may face long-term health complications.
Q: Can the baby’s gender be determined during the timeline of organ development?
A: Yes, the baby’s gender can be determined during the timeline of organ development. The genitalia start to form around week 9-12 of pregnancy, which can be seen on an ultrasound.
Q: Are all babies born with the same size organs?
A: No, all babies are not born with the same size organs. The size of the baby’s organs can vary based on genetics, prenatal environment, and other factors.
Q: Can a baby’s organ development be affected by genetics?
A: Yes, a baby’s organ development can be affected by genetics. Some genetic disorders can impact the development of certain organs.
In conclusion, understanding the timeline of when the major organs develop in a baby can help expecting parents take care of themselves and their baby during pregnancy. It is important to consult with your healthcare provider if you have any concerns about your baby’s development.