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When Does An Embryo Become A Fetus?

When Does An Embryo Become A Fetus?

Feb 12, 2022


Dr. Jacqueline Darna

Today you will learn the science behind the journey your baby takes from fertilization to embryo to a fetus, and finally discuss what every pregnant mom wants to know “is my baby viable, or can they live outside the womb?” All coming up in today’s Pregnancy Pukeology Podcast Episode 74: When does an embryo become a fetus.

Listen to "When Does An Embryo Become A Fetus? Growing Babies Journey! Pregnancy Podcast Pukeology Ep. 74" on Spreaker.


pregnancy embryo fetus when how long


Babies’ growth always start in utero from just fertilization to the point of birth. When the female ovum combines with the male sperm a fertilization occurs to form the first stage in utero which is a zygote. The fertilization occurs in the fallopian tube, this is followed by the movement of the zygote to the uterus for implantation, the zygote undergoes multiple division to form blastomeres which at stage of 16 to 32 blastomeres form a morula. This develops a fluid filled cavity to form the blastocyst. 

At around 3 weeks, the developing baby is called an embryo. The embryo stage lasts from week 3 to week 8 and this stage marks the major stage of organ development in the developing child. Therefore any teratogens which the baby may be exposed to at this level may have very severe negative effects on the baby. 

After 8 weeks, the baby enters a stage called the fetal stage which lasts until birth. 


The developing fetus require full development of the different organs to ensure all the body systems work properly. Embryonic period always serve the purpose of organogenesis and this the time the baby is between 3weeks to 8 weeks in utero. 

Cardiovascular system development start around the 3rd week with primordial heart development. This continues to septum formation at around 7th week 

. By around 10th to 12th week, the heartbeat of the fetus can be felt by use of an ultrasound method called Doppler ultrasound. 

The respiratory and digestive system start as one tube initially at around the third week and later divide to form the respiratory tract and the digestive tract. By 24 weeks the lungs has surfactant, a factor which is very important for babies breathing. 

The nervous system form from the neural plate, which later forms the neural tube responsible for forming both the brain and the spinal cord. The peripheral nervous system also develop from the neural plate which has cranial and caudal portion. 

The different glands of the child are also formed during the embryonic period. These glands include the liver, thyroid and parathyroid glands. 

Formation of the bones and other musculoskeletal system starts by 12 week but continues to even after birth. This includes the ossification of bones. 

The end of embryonic period results in organs having formed but most of this organs are in the primordial stage and development continues in the fetal period to ensure the organs are mature and fully developed. 

Fetal development stages and timeline of fetal development 

The fetal period lasts from 8 weeks up to term. Remember, by the end of embryonic period, organogenesis is complete and the fetus is about 2.5 cm in dimension. The fetal period is marked by development of the fetus both in the organs and this is shown by increase in length and weight. The organs also develop into more complex and functional systems. 

By 12 weeks gestation, the baby may start moving, the expected weight is 45grams and the fetus is around 8 cm in length. At this time, the kidney have started functioning by producing urine. 

By 20 weeks, there are spontaneous fetal movements, the baby is continuing to grow at a weight of 225grams and length of 25cm. 

When is my baby viable? 

 The fetus starts to be viable by around 24 weeks though this age may differ from different countries depending on the level of medical expertise and equipments. 

The baby continues to grow such that by term which is 40 weeks, the weight is about 3kg and length is 48 to 52cm. At this point the baby is term and ready for birth.  






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