Pregnancy is a thrilling, but sometimes overwhelming experience, and knowing how your baby is changing and developing week by week can make you feel more connected and confident throughout this precious time. Your baby begins its amazing journey of growth and development from the moment they are conceived.
Fetal Weight Chart by Week
Here’s an overview of fetal weight changes during each week of pregnancy:
| Week | Fetal Weight |
| Week 1-2 | Not applicable yet |
| Week 3-4 | Very tiny, barely measurable |
| Week 5 | 0.04 oz (1 gram) |
| Week 6 | 0.07 oz (2 grams) |
| Week 7 | 0.14 oz (4 grams) |
| Week 8 | 0.25 oz (7 grams) |
| Week 9 | 0.5 oz (14 grams) |
| Week 10 | 0.7 oz (20 grams) |
| Week 11 | 0.9 oz (25 grams) |
| Week 12 | 1 oz (28 grams) |
| Week 13 | 1.2 oz (35 grams) |
| Week 14 | 1.6 oz (45 grams) |
| Week 15 | 2.1 oz (60 grams) |
| Week 16 | 2.8 oz (80 grams) |
| Week 17 | 3.5 oz (100 grams) |
| Week 18 | 4.2 oz (120 grams) |
| Week 19 | 5 oz (140 grams) |
| Week 20 | 7 oz (200 grams) |
| Week 21 | 9 oz (250 grams) |
| Week 22 | 11 oz (300 grams) |
| Week 23 | 12 oz (340 grams) |
| Week 24 | 14 oz (400 grams) |
| Week 25 | 1 lb (450 grams) |
| Week 26 | 1.5 lbs (680 grams) |
| Week 27 | 2 lbs (900 grams) |
| Week 28 | 2.3 lbs (1 kg) |
| Week 29 | 2.5 lbs (1.1 kg) |
| Week 30 | 3 lbs (1.4 kg) |
| Week 31 | 3.3 lbs (1.5 kg) |
| Week 32 | 3.7 lbs (1.7 kg) |
| Week 33 | 4 lbs (1.8 kg) |
| Week 34 | 4.5 lbs (2 kg) |
| Week 35 | 5 lbs (2.3 kg) |
| Week 36-40 | 5.5-7 lbs (2.5-3.2 kg) |
| Week | Baby Growth & Weight | Development Milestones |
| Week 1-2 | Baby not formed yet; early stage of fertilization. | The egg is fertilized and begins to implant in the uterus. |
| Week 3 | Size: Pinhead | The fertilized egg is now an embryo, and implantation begins. |
| Week 4 | Size: Small pea (0.1 inches) | The embryo forms the neural tube that will become the brain and spinal cord. |
| Week 5 | Size: 0.25 inches | Early development of the heart and circulatory system. The heart begins to beat. |
| Week 6 | Size: 0.5 inches | Formation of eyes and ears. Limb buds start to appear. |
| Week 7 | Size: 0.6 inches | Arm and leg buds elongate. The heart is now fully formed and beating strongly. |
| Week 8 | Size: 0.7 inches | All organs are beginning to form. The baby’s face and features start to take shape. |
| Week 9 | Size: 1.2 inches | The baby is now called a fetus. Organ development continues, and external genitalia are forming. |
| Week 10 | Size: 1.4 inches | Fingers and toes are fully formed, although webbed. The baby now moves, though it’s still too small to be felt. |
| Week 11 | Size: 1.6 inches | The baby is starting to develop distinct human features, and kidneys begin to function. |
| Week 12 | Size: 2 inches | Baby’s vocal cords and digestive system continue to develop. The baby may start sucking its thumb. |
| Week 13 | Size: 2.5 inches | The baby’s facial muscles start to work, and hair begins growing. |
| Week 14 | Size: 3.4 inches | Fine hair (lanugo) starts to cover the baby’s body. Muscles develop, and joints are working. |
| Week 15 | Size: 4 inches | Baby begins to develop a sense of touch and may react to external stimuli. |
| Week 16 | Size: 4.6 inches | The baby’s heart is beating in a regular rhythm. The skin is still thin and transparent. |
| Week 17 | Size: 5 inches | Lanugo (fine hair) covers the baby’s body, and the baby starts to practice swallowing amniotic fluid. |
| Week 18 | Size: 5.5 inches | Baby begins to react to light and can make small movements like sucking its thumb. |
| Week 19 | Size: 6 inches | Baby can now hear sounds from the outside world. Eyebrows and eyelashes are forming. |
| Week 20 | Size: 10.5 inches | Midpoint of pregnancy, the baby starts to make more noticeable movements. Organs and tissues continue developing. |
| Week 21 | Size: 10.5 inches | Baby’s taste buds are functioning. The baby’s lungs are developing, but they are not yet fully functional. |
| Week 22 | Size: 11 inches | Baby’s eyes open and blink. Baby starts to gain weight rapidly. |
| Week 23 | Size: 11.5 inches | Baby is starting to build fat layers and muscles. The body is filling out. |
| Week 24 | Size: 12 inches | Baby’s lungs are maturing, and they start producing a substance (surfactant) that helps the lungs inflate. |
| Week 25 | Size: 12.5 inches | Baby’s skin becomes smoother as fat deposits under the skin. It starts gaining more weight. |
| Week 26 | Size: 13 inches | Baby’s eyes begin to open and close. Baby is now moving actively inside the womb. |
| Week 27 | Size: 13.5 inches | Brain development continues rapidly. Baby begins practicing breathing movements. |
| Week 28 | Size: 14 inches | Baby’s bones are stronger, and the baby is starting to gain more fat. Lungs are still immature but developing. |
| Week 29 | Size: 15 inches | The baby’s brain is developing rapidly, and the bones are becoming more solid. |
| Week 30 | Size: 15.7 inches | Baby's lungs continue to develop. The body is filled with fat for warmth after birth. |
| Week 31 | Size: 16 inches | Baby's brain is becoming more sophisticated. Baby continues to practice breathing and moving. |
| Week 32 | Size: 16.5 inches | Fat continues to accumulate under the skin, and the baby starts gaining weight quickly. |
| Week 33 | Size: 17 inches | Baby’s eyes are fully formed, and they can blink. Baby’s skin becomes less wrinkled. |
| Week 34 | Size: 17.7 inches | The baby’s lungs are continuing to mature and the baby’s body becomes rounder. |
| Week 35 | Size: 18 inches | Baby is gaining more weight. The skin becomes smooth, and the baby is now considered "full term." |
| Week 36 | Size: 18.5 inches | Baby’s body is more plump, and it has less room to move around. It continues gaining weight. |
| Week 37-40 | Size: 19-21 inches | Baby is full-term, and the organs are fully developed. Baby continues gaining weight in preparation for birth. |
Key Milestones According To Trimester
- First Trimester (Week 1-12): The embryo develops organs and the basic body structure. By week 12, the baby has recognizable human features.
- Second Trimester (Week 13-26): The baby grows rapidly, and movements become noticeable. External features like hair and skin continue to develop.
- Third Trimester (Week 27-40): The baby focuses on gaining weight and strength, preparing for life outside the womb. The lungs and organs continue maturing.
Conclusion
As your baby develops, it's incredible to see how rapidly development occurs. Every week sees new changes, and knowing these developments can make the experience even more special. Right from the initial phases when your baby is merely a small cluster of cells to the last few weeks when they are nearly ready for birth, each phase is important for your baby's development and health. Take note of these milestones, and cherish every part of your pregnancy. It is a special and magical time for you and your baby!
