Fetal Development Week by Week: A Visual Guide to Your Baby's Growth
See your baby's amazing journey! This week-by-week visual guide explains fetal development with a size and weight chart, from a tiny embryo to a full-term baby.
Pregnancy is a 40-week miracle, transforming a single fertilized egg into a fully formed, unique little person. Watching this progress is one of the most magical parts of the journey.
This visual guide will walk you through your baby's development, week by week, so you can understand the incredible milestones happening inside you.
A Quick Note on "Gestational Age"
Before we begin, it's important to understand how pregnancy is measured. Your doctor will calculate your 40-week timeline using Gestational Age, which starts from the first day of your last menstrual period (LMP).
This means that during "Weeks 1 and 2," you aren't actually pregnant yet! Your body is just preparing for ovulation. Conception typically happens around "Week 3." The "fetal age" (your baby's actual age) is about two weeks behind the gestational age.
All the weeks in this guide refer to the gestational age, which is the standard used by all medical professionals.
Fetal Growth Chart: Average Length & Weight
Here is a visual, at-a-glance chart to track your baby's growth. Length is measured from the top of the head to the heel (crown-to-heel), unless noted.
| Week | Average Length (inches) | Average Weight (ounces) | Common Size Analogy |
|---|---|---|---|
| Week 8 | 0.6 in (crown-to-rump) | 0.04 oz | Raspberry |
| Week 12 | 2.1 in (crown-to-rump) | 0.5 oz | Plum |
| Week 16 | 4.6 in | 3.5 oz | Avocado |
| Week 20 | 10.1 in | 10.6 oz | Banana |
| Week 24 | 11.8 in | 1.3 lbs | Ear of Corn |
| Week 28 | 14.8 in | 2.2 lbs | Eggplant |
| Week 32 | 16.7 in | 3.7 lbs | Jicama |
| Week 36 | 18.7 in | 5.8 lbs | Romaine Lettuce |
| Week 40 | 20.2 in | 7.6 lbs | Small Pumpkin |
Note: All sizes and weights are averages. Every baby grows at their own unique pace.
Table of Contents
- The First Trimester: Weeks 1-13
- The Second Trimester: Weeks 14-27
- Week 14: Grimacing
- Week 15: Skeleton Hardening
- Week 16: Blinking
- Week 17: Fat Stores
- Week 18: Hearing Sounds
- Week 19: Vernix Caseosa
- Week 20: Halfway There!
- Week 21: Eyebrows
- Week 22: Lanugo
- Week 23: Sense of Motion
- Week 24: The Viability Milestone
- Week 25: Responding to Your Voice
- Week 26: Developing Lungs
- Week 27: Brain Activity
- The Third Trimester: Weeks 28-40
- Week 28: Eyes Open
- Week 29: Full Bone Structure
- Week 30: Red Blood Cells
- Week 31: Rapid Weight Gain
- Week 32: Practicing Breathing
- Week 33: Detecting Light
- Week 34: Lungs Maturing
- Week 35: Nearly Ready
- Week 36: "Dropping"
- Week 37: Early Term
- Week 38: Shedding Lanugo
- Week 39: Full Term!
- Week 40: Your Due Date
- After Your Due Date: Weeks 41-42
The First Trimester: Weeks 1-13
This is the "blueprint" phase. Your baby goes from a single cell to a fully-formed fetus with all major organ systems in place.
Weeks 1-3: Getting Started
- What's happening: Week 1 is your period. In Week 2, your body ovulates. In Week 3, fertilization occurs, and the rapidly dividing ball of cells (called a blastocyst) travels to your uterus and implants in the lining.
Week 4: Implantation
- Size: Poppy Seed
- Development: The blastocyst is now an embryo. The neural tube, which will become the brain and spinal cord, is beginning to form. The placenta has also begun to form.
Week 5: A Heartbeat Begins
- Size: Apple Seed
- Development: The baby's heart begins to form and starts to beat, pumping blood. Major organs, including the brain, are taking shape.
Week 6: Taking Shape
- Size: Sweet Pea
- Development: The neural tube closes. Facial features are beginning to form, with dark spots where eyes will be and pits for ears. Small buds that will become arms and legs are visible.
Week 7: Arms and Legs
- Size: Blueberry
- Development: The arm and leg buds are growing longer, and hands and feet are starting to form, looking like tiny paddles. The brain is developing rapidly.
Week 8: Now a Fetus
- Size: Raspberry
- Development: The embryo is now officially called a fetus! Webbed fingers and toes are forming. The lungs and digestive system are continuing to develop.
Week 9: Major Milestones
- Size: Cherry
- Development: The baby's basic anatomy is in place. The heart has four chambers, and tiny valves are forming. The tail-like structure at the end of the spine has disappeared.
Week 10: Making a Fist
- Size: Strawberry or Prune
- Development: The baby's arms can bend at the elbow and they can make a tiny fist. Tooth buds are forming inside the mouth.
Week 11: Bones Harden
- Size: Lime
- Development: The fetus is fully formed. Bones are beginning to harden (ossify). The diaphragm is forming, and the baby may be having hiccups, though it's far too early for you to feel them.
Week 12: Vocal Cords
- Size: Plum
- Development: Vocal cords are forming, and the kidneys are able to make urine. The baby's face looks much more human.
Week 13: Fingerprints
- Size: Peach or Lemon
- Development: Tiny, unique fingerprints are now forming on the baby's fingers. If you're having a girl, her ovaries already contain millions of eggs. You've reached the end of the first trimester.
The Second Trimester: Weeks 14-27
This is the "growth and detail" phase. Your baby will grow dramatically in size, and all their systems will become more refined.
Week 14: Grimacing
- Size: Apple
- Development: The baby can now squint, grimace, and frown. The liver is making bile, and the spleen is helping to produce red blood cells.
Week 15: Skeleton Hardening
- Size: Navel Orange
- Development: The baby's skeleton is hardening and becoming visible on ultrasound. The baby can sense light that filters through your abdomen.
Week 16: Blinking
- Size: Avocado
- Development: The eyes can blink! The nervous system is starting to function. Your baby's movements are becoming more coordinated.
Week 17: Fat Stores
- Size: Pear
- Development: Fat stores, which are essential for heat regulation, are beginning to develop under the skin. The umbilical cord is growing thicker and stronger.
Week 18: Hearing Sounds
- Size: Bell Pepper
- Development: The baby's ears are in their final position and they can likely hear sounds, including your heartbeat, your voice, and muffled sounds from the outside world.
Week 19: Vernix Caseosa
- Size: Heirloom Tomato
- Development: A waxy, cheese-like coating called vernix caseosa is forming on the skin. This protects your baby from the amniotic fluid.
Week 20: Halfway There!
- Size: Banana (in length)
- Development: You've reached the halfway point! You will likely have your anatomy scan ultrasound around this time. You may also begin to feel the baby's first distinct movements, known as "quickening."
Week 21: Eyebrows
- Size: Pomegranate
- Development: Eyebrows and eyelids are now present. If you're having a girl, her vagina has begun to form.
Week 22: Lanugo
- Size: Papaya
- Development: The baby's skin is covered in fine, downy hair called lanugo. This hair, along with the vernix, protects the delicate skin.
Week 23: Sense of Motion
- Size: Grapefruit
- Development: The inner ear is now fully developed, giving the baby a clear sense of motion. They can feel you move, dance, or walk.
Week 24: The Viability Milestone
- Size: Ear of Corn (in length)
- Development: This is a major milestone. The baby's lungs are developing "surfactant," a substance that will help the air sacs inflate after birth. Babies born at this week may be able to survive with intensive medical care.
Week 25: Responding to Your Voice
- Size: Acorn Squash
- Development: The baby's startle reflex is developing. They may jump at a loud noise or even be soothed by the sound of your voice.
Week 26: Developing Lungs
- Size: Zucchini (in length)
- Development: The lungs are continuing to mature, and the baby is practicing "breathing" by inhaling and exhaling amniotic fluid.
Week 27: Brain Activity
- Size: Head of Cauliflower
- Development: The baby's brain is very active now. While they still sleep most of the day, their brain is developing the complex connections needed for life outside the womb. You've reached the end of the second trimester.
The Third Trimester: Weeks 28-40
This is the "finishing touches" phase. Your baby's main job is to gain weight and mature their brain and lungs in preparation for birth.
Week 28: Eyes Open
- Size: Large Eggplant
- Development: The baby's eyes can open, close, and blink. Their eyesight is still developing, but they can perceive light and darkness.
Week 29: Full Bone Structure
- Size: Butternut Squash
- Development: The baby's bones are now fully developed but are still soft and pliable, which will help with the journey through the birth canal.
Week 30: Red Blood Cells
- Size: Head of Cabbage
- Development: Your baby's bone marrow has taken over the job of producing red blood cells. They are gaining about half a pound per week.
Week 31: Rapid Weight Gain
- Size: Coconut
- Development: The baby is rapidly gaining protective fat, which makes their skin look less wrinkled and more smooth. All five senses are functional.
Week 32: Practicing Breathing
- Size: Jicama
- Development: The lanugo (fine body hair) is starting to fall off. The baby is practicing breathing, swallowing, and sucking—all skills they'll need right after birth.
Week 33: Detecting Light
- Size: Pineapple
- Development: The baby's pupils can now constrict and dilate in response to light. They are likely in the head-down position in preparation for birth.
Week 34: Lungs Maturing
- Size: Cantaloupe
- Development: The baby's lungs and central nervous system are continuing to mature. The vernix coating on their skin is getting thicker.
Week 35: Nearly Ready
- Size: Honeydew Melon
- Development: The lungs are nearly fully mature. The baby is gaining significant weight (over 1.5 lbs this month alone) and has less room to do flips, but you'll still feel plenty of strong kicks and rolls.
Week 36: "Dropping"
- Size: Head of Romaine Lettuce (in length)
- Development: The baby may begin to "drop" or "engage" into your pelvis, a process called lightening. This takes pressure off your lungs (making it easier to breathe) but puts more pressure on your bladder.
Week 37: Early Term
- Size: Bunch of Swiss Chard (in length)
- Development: Your baby is now considered "Early Term." They are fully developed, but these last few weeks are crucial for brain and lung maturation.
Week 38: Shedding Lanugo
- Size: Leek (in length)
- Development: The baby is shedding most of the lanugo and vernix. They are also producing meconium (their first bowel movement) in their intestines.
Week 39: Full Term!
- Size: Small Watermelon
- Development: Your baby is officially "Full Term"! Their brain is still growing at an astonishing rate. They are ready for life on the outside.
Week 40: Your Due Date
- Size: Small Pumpkin
- Development: This is it! But remember, the due date is just an estimate. Only about 5% of babies are born on this exact day.
After Your Due Date: Weeks 41-42
If your baby hasn't arrived by Week 41, you are "Late Term." If you go past Week 42, you are "Post-Term." Your doctor will be monitoring you and your baby very closely at this stage and may discuss inducing labor to ensure a safe delivery.
Medical Disclaimer
The information in this article, including all sizes, weights, and milestones, is for educational and informational purposes only. It is intended as a general guide and is not a substitute for professional medical advice. Every pregnancy and every baby is unique and will grow at their own pace. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding your pregnancy.
About the Author
Abhilasha Mishra is a health and wellness writer specializing in women's health, fertility, and pregnancy. With a passion for empowering individuals through evidence-based information, she writes to make complex health topics accessible and actionable.