A developing baby's cells start the incredible journey of becoming organs before the embryo implants in its mother's womb. From then on, just ten weeks is all it takes for a full set of vital organs to form — yet they still require further development, which continues through pregnancy. By 37 weeks, these tissues and systems will have fully matured. Let's explore the fascinating world of your baby's organ development.
Around the time of implantation in the wall of the mother’s womb, the cells in an embryo start forming different organs.
All the essential organs are present in their basic forms by ten weeks of pregnancy. They will develop and mature throughout pregnancy. Some continue to grow after birth.
At 24 weeks, the organ systems have developed enough that a baby has a 50% chance of surviving outside the womb with support.
A baby’s organs have fully developed at 37 weeks. Changes in vision, hearing, the ability to suck and swallow, and brain growth happen between weeks 37–40 to help improve health outcomes after birth.
When do fetal organs begin to develop?
The cells in a developing baby start becoming separate organs around day 8–9 after conception, as the embryo implants in the wall of the mother’s womb.
At conception — when a sperm fertilizes the egg — the first cell that will grow into a baby is called a zygote. The zygote begins to divide rapidly after about 24–36 hours. Once there are eight cells, they begin to change and develop characteristics of the organs that they will become. The cell ball is now called a blastocyst. While it grows, it travels to where it will implant inside the womb.
Three layers of cells are present, starting the process of creating:
- Skin and nervous system
- Digestive system and lungs
- Muscles and skeleton
Early miscarriage often happens if the embryo cannot continue development because of a problem with the egg or sperm. About half of the embryos that are conceived do not survive this stage. They pass out of the body without implantation, and a mother may not even miss her menstrual cycle or know she is pregnant.
From weeks 3–9, called the embryonic stage, the baby will change from this mass of cells to distinctly human looking. During this stage, the following developments occur:
- Hands and feet. Fingers and toes begin to form. They are webbed at first.
- Facial muscles. The muscles that make up the face form.
- Respiratory system. The lungs and tubes that air travels through after birth form.
- Hearing. The parts of the inner ear that lets the baby hear begin to grow.
When are all baby organs present?
After eight weeks of incredible growth and development, the embryo has all the essential organs and parts except those connected with gender. It is only an inch long and weighs less than one gram. Because so much growth happens so quickly, it is crucial that pregnant mothers avoid substances that could interfere with organ development at this point in pregnancy.
After ten weeks, all organs are present, though some need more development. The brain and spinal cord are present, but will continue to develop until birth. The brain will continue to grow new cells for an entire year after the baby’s birth.
In weeks 9–12 the following developments occur:
- Nails. The baby grows fingernails and toenails.
- Cartilage. Cartilage forms and brings structure to the limbs.
- Urine. The kidneys start making urine.
- Pancreas. The pancreas begins making insulin to process sugar.
When does the baby’s heart develop?
The baby’s heart begins to pump fluid through blood vessels on day 20, and its red blood cells form starting on day 21.
While it pumps blood, it grows and changes into a mature, four-chambered heart. These four heart chambers form by the end of week 7.
The heartbeat can be detected by ultrasound sometimes as early as 5 and a half weeks of gestation and more reliably by 6–7 weeks. Sometimes a very early ultrasound can fail to find the heartbeat, which can be terrifying for new parents. Keep in mind that it may just be a little too early to hear the heartbeat yet. When a healthcare team member can find the heartbeat with an external doppler depends on a few things:
- The way the baby is lying inside the womb
- Whether the mother is overweight
- The placement of the placenta
It is possible in ideal conditions to find the heartbeat by doppler as early as nine weeks. More often, it takes until weeks 10–12.
When are the lungs ready?
The lungs are not ready to function fully outside the womb until 36 or 37 weeks.
From week 5, the lungs begin to form, starting as tiny buds and growing rapidly. The baby practices breathing movements as soon as there are lung buds and will continue to breathe amniotic fluid in and out until birth.
Between weeks 13–17, the lungs start making alveoli — the air sacs that exchange oxygen. By week 25, the lungs are fully formed but not ready to function outside the uterus without support. They start making a fluid called surfactant they need to help with breathing movements between 25 and 28 weeks.
When babies are born at term, they have developed 20–50 million alveoli. They will continue to develop more throughout childhood and into their teenage years until they have 300 million as adults.
Preterm birth survival statistics
At 24 weeks, babies reach viability — meaning they have over a 50% chance of surviving outside the womb with support. Before 24 weeks, babies have a low probability of survival. Every day inside the womb counts to help your baby have better outcomes. If your water breaks, or you go into labor early, doctors will try to delay preterm birth for as long as possible.
|Time||Chance of survival|
|24 weeks||Before 24 weeks, babies have a low probability of survival; After 24 weeks, there's a 50% chance of survival.|
|28 weeks||Babies born at 28 weeks have an 80–90% chance of survival; However, there's a 10% chance of having long-term health problems.|
|34 weeks||At 34 weeks gestation, babies have the same chance of survival as full-term babies.|
When is a baby ready to be born?
Doctors consider a baby’s organs fully developed at 37 weeks, but do not recommend starting labor by induction until 39 weeks unless the mother’s or baby’s health is at risk.
Although the organs have completed their essential development, many things happen at the end of pregnancy to help the baby do well outside the womb.
- Brain. The brain grows in volume by a third between 35 and 39–40 weeks.
- Lungs. The more the lungs develop, the better the baby can breathe outside the womb.
- Hearing. Babies born too early are more likely to have hearing problems.
- Vision. Babies born too early are more likely to have issues with vision.
- Suckling. Babies are still learning to suck and swallow– babies born at term have an easier time feeding than babies born preterm.
There is also a risk that a mother will need a c-section if a doctor induces labor before her body is ready for it.
During pregnancy, a baby changes quickly from a single fertilized cell to a full human with functioning organs. It is crucial to avoid hazards and get the proper nutrients to support your baby’s development during this amazing time.
- American Journal of Obstetrics and Gynecology. Human fetal growth and organ development: 50 years of discovery.
- Seminars in Perinatology. Benefits and risks of ultrasound in pregnancy.