Table of Contents
Introduction
Fetal development is a continuous, dynamic process that begins at conception and ends at birth. Pregnancy is a fascinating journey filled with rapid changes, such as tiny cells multiplying, organs taking shape, and the senses awakening, all working together to create a healthy baby.
Whether you’re a first-time mom or a healthcare student eager to learn, following the week-by-week stages of fetal development can help make sense of what’s happening and when.
This comprehensive guide also addresses key questions such as when organs form in the fetus, how stress or nutrition affects the baby, and when you can expect to feel the first kick or hear that little heartbeat.
Let’s start with the broader picture, the three trimesters of pregnancy.
The 3 Trimesters of Pregnancy
First Trimester
The first trimester (weeks 1-8) is a time of foundational development. After fertilization, the zygote implants into the uterine lining and rapidly divides into specialized cells.
By week 5, the embryo has a beating heart. Organs such as the brain, liver, and kidneys begin forming by week 8.
This period is critical because many major organs begin to form. Disruptions from infections, medications, or poor nutrition during this window can interfere with organ development.
Second Trimester
Often called the golden period, the second trimester (weeks 13-27) marks when most women feel more energetic and fetal movement becomes detectable. Around week 16, some parents may feel the baby move, known as quickening.
Sensory systems start forming, and by week 20, many expectant parents discover their baby’s sex during an ultrasound. Taste buds develop, and the fetus begins swallowing amniotic fluid.
The skin becomes more structured, and fine hair called lanugo appears. Baby growth during pregnancy during this trimester is steady and noticeable.
Third Trimester
In the final stretch (weeks 28-40), the fetus gains most weight and prepares for life outside the womb. The lungs mature, the brain develops significantly, and fat stores build.
Babies begin responding to light and sound. Kicks may feel stronger and more frequent as space tightens.
If babies are born after week 37, they are considered full-term. By week 40, the fetus is typically 19 to 21 inches long, weighing between 6 and 9 pounds—about watermelon size.
Week-by-Week Fetal Development
Week 8
Although the embryo is only about the size of a raspberry, tiny fingers and toes are starting to emerge. Eyelids cover the eyes, and the major internal organs continue forming. Brain activity begins, and limb movement is possible, though not yet felt by the mother.
Week 12
Nearly all major organs have formed by the end of the first trimester. The intestines are moving into their proper position, and fingernails begin to grow. The fetus can now make sucking movements and may respond to stimuli like touch.
Week 20
At this point, your baby is about the size of a banana. A detailed anomaly scan can usually reveal the sex of the baby and the formation of facial features. The skin is still translucent but becoming less so. The fetus can now hear and may react to loud noises. This period is a crucial time in the fetal development timeline.
Week 24
Although still premature, babies born around week 24 now have a higher chance of survival with neonatal care. The lungs produce surfactant, which keeps the air sacs open. The brain and sensory organs continue to grow rapidly.
Week 30
The fetus shows periods of deep and light sleep, sometimes even experiencing REM sleep. Movement becomes more rhythmic, and you might notice patterns in kicks. The baby is now about 15.7 inches long, roughly the size of a cabbage.
Week 36–40
By week 36, the baby typically moves into a head-down position. Most of the lanugo has shed, and the lungs are ready for breathing. The skull remains soft and flexible for birth. At week 40, your baby is fully developed and prepared to meet the world.
Factors Affecting Fetal Development
Nutrition Do's & Don'ts
A nutrient-rich diet supports optimal fetal growth. Key nutrients include:
- Folic Acid, which is crucial for neural tube development
- Iron, which promotes red blood cell production
- Calcium & Vitamin D, which is essential for bone development
- Omega-3 fatty acids (like DHA), which support brain and eye development
Avoid high-mercury fish (e.g., swordfish), unpasteurized dairy, and excess caffeine. Alcohol and recreational drugs can significantly impair fetal brain and organ development.
Environmental Risks to Avoid
- Smoking: Associated with low birth weight and developmental delays
- Radiation or toxic chemical exposure: Avoid potent cleaning agents, pesticides, and lead-containing products
- Infections: TORCH infections (toxoplasmosis, rubella, cytomegalovirus, and herpes) can affect development. Get vaccinated before or early in pregnancy, as advised.
Stress and sleep deprivation also influence fetal health. Chronic stress can trigger hormonal changes that may impact fetal growth and brain development.
Frequently Asked Questions
The fetal heartbeat usually begins by the end of week 5 or early in week 6. It may be visible on an ultrasound before the mother can hear it.
Yes. Elevated maternal cortisol (the stress hormone) levels may cross the placenta and influence fetal brain development, especially if sustained over time. Stress-reduction techniques such as prenatal yoga, therapy, and adequate rest are essential.
Hearing develops around week 18 but becomes more refined by week 25. By the third trimester, babies may respond to familiar voices or music.
Final Thought for You
Watching a baby grow during pregnancy is nothing short of amazing. It’s a journey shaped by nature, nourished by what we eat, and affected by everything around us. Week by week so much happens — tiny organs begin to form, and before long, your baby may even start to recognize the sound of your voice.
Staying informed about the stages of fetal development helps you make better health decisions for yourself and your baby.
To support this journey, choose high-quality prenatal supplements like organic prenatal vitamins, eat nutrient-rich foods, and avoid known risks.
Whether you’re a healthcare student wanting to learn more or a parent-to-be excitedly tracking each week, this guide gives you a trustworthy look at how life grows inside the womb.
Remember that every pregnancy is different, and regular checkups are essential to help mom and baby stay healthy.
O’Dowd MJ, O’Dowd TM. Quickening–a re-evaluation. Br J Obstet Gynaecol. 1985;92(10):1037-1039.
Getz, M. How does your baby eat and poop in the womb? BabyCenter.
Pascual ZN, Langaker MD. Physiology, pregnancy. In: StatPearls. StatPearls Publishing; 2025.
Donovan MF, Cascella M. Embryology, Weeks 6-8. In: StatPearls. StatPearls Publishing; 2025.
Han S, Mallampalli RK. The role of surfactant in lung disease and host defense against pulmonary infections. Annals of the American Thoracic Society. 2015;12(5):765.
Verhave BL, Nassereddin A, Lappin SL. Embryology, lanugo. In: StatPearls. StatPearls Publishing; 2025.
NHS: 18 weeks pregnant – Week-by-week guide. nhs.uk.
