One of the most incredible and fascinating aspects of pregnancy is the development of the fetus. From the moment of conception, a tiny ball of cells begins to grow and evolve into a fully formed baby over the course of 40 weeks. Understanding the stages of fetal development week by week can give expectant parents valuable insight into the growth and changes their baby is experiencing.
Weeks 1-4: Conception and Implantation
During the first few weeks of pregnancy, the fertilized egg travels down the fallopian tube and implants itself into the uterine lining. This process marks the beginning of fetal development, as the cells begin to divide and grow rapidly. By the end of week 4, the tiny embryo is about the size of a poppy seed.
Weeks 5-8: Embryonic Stage
During weeks 5-8, the embryo grows rapidly and begins to develop the basic structures of the body. By the end of this stage, the heart begins to beat, and the major organs and systems, such as the brain, spinal cord, and digestive system, start to form. The embryo is now referred to as a fetus.
Weeks 9-12: First Trimester
By weeks 9-12, the fetus has grown to about the size of a lime and is more recognizable as a tiny human. Facial features begin to develop, and the fingers and toes become distinct. The baby’s movements can be detected during an ultrasound, although the mother may not feel them yet.
Weeks 13-16: Second Trimester Begins
During weeks 13-16, the fetus continues to grow and develop rapidly. The eyes move to the front of the face, and the ears are in their final position. The baby’s skin is transparent and covered in fine hair called lanugo. By the end of the 16th week, the fetus is about the size of an avocado.
Weeks 17-20: Halfway Point
At the halfway point of pregnancy, the fetus is about the size of a banana and is growing quickly. The baby’s bones are becoming harder, and the skin is starting to develop a protective coating called vernix. By week 20, the mother may start to feel the baby’s movements, known as quickening.
Weeks 21-24: Viability
During weeks 21-24, the fetus is considered viable, meaning that it has a chance of surviving outside the womb with medical intervention. The baby’s lungs are developing, and the brain is growing rapidly. By week 24, the fetus is about the size of a cantaloupe.
Weeks 25-28: Third Trimester Begins
As the third trimester begins, the fetus is rapidly gaining weight and growing in size. The baby’s senses, such as hearing and touch, are becoming more developed. The lungs are maturing, and the baby is practicing breathing movements. By week 28, the fetus is about the size of an eggplant.
Weeks 29-32: Final Stretch
During weeks 29-32, the fetus is nearing its final size and weight. The bones are continuing to harden, and the baby is moving frequently. The baby’s skin is turning from transparent to pink, and the eyes are opening and closing. By week 32, the fetus is about the size of a squash.
Weeks 33-36: Preparation for Birth
As the due date approaches, the fetus is running out of space in the womb. The baby may drop lower into the pelvis in preparation for birth, known as engagement. The baby is gaining weight rapidly, and the lungs are almost fully developed. By week 36, the fetus is about the size of a honeydew melon.
Weeks 37-40: Full-Term
By weeks 37-40, the fetus is considered full-term and ready to be born. The baby is gaining weight and adding layers of fat for insulation. The fetus is now fully developed and is about the size of a watermelon. The mother may experience signs of labor, such as contractions and the rupture of the amniotic sac.
Understanding fetal development week by week can help expectant parents appreciate the amazing journey their baby is on as it grows and develops in the womb. From the tiny embryo to the fully formed baby ready to enter the world, each stage of development is truly a miracle.