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Fetal Development Timeline

  • Week 1: Last Menstrual Period

    Week one is the week of your last menstrual period. The first day of menstrual bleeding is considered the official start day of pregnancy. Pregnancy is counted from the first day of your last menstrual period to birth which is about 40 weeks
  • Week 2: Fertilization

    Week 2: Fertilization
    Every month 3-10 eggs receive a hormonal signal the cues them to mature and prepare for ovulation. At the same time the uterus is forming the endometrium. In one of the ovaries the eggs ripen in fluid filled sacs called follicles. When the sperm and the egg join in the Fallopian tube fertilization has occurred
  • Week 3: Cells Join, Divide - New Life Begins

    Week 3: Cells Join, Divide - New Life Begins
    The fertilized egg immediately begins to divide and float down into the uterus. Immediately upon fertilization your little one is set as a boy or a girl. The sex of your child is predetermined by the fathers sperm chromosomes. The cell contains all of the DNA to become a child, baby begins to form from this single fertilized egg. Cell divides itself into 2 cells, then 4, 8, and so on rapidly
  • Week 4: Implantation

    Week 4: Implantation
    Fertilized egg is now called a blastocyst, it is a fluid filled cluster of 50-60 cells, still multiplying. Implantation occurs about day 5 to day 8 of embryo development. Embryo is only one-hundredth of an inch long. Site of implantation is where the placenta attaches to the uterine wall. After blastocyst has implanted, it will release a hormone called human gonadotropin hormone (hCG) into the blood stream
  • Week 5: Embryonic Stage Begins

    Week 5: Embryonic Stage Begins
    Amniotic fluid building up, embryo grows from 2mm, by end of week embryo has doubled in size to about 4-5mm, egg has now completely grown into the mucous membrane of the uterus, a fibrin clot closes hole to uterus cavity, placenta is developing, embryo starts receiving oxygen and nutrition through the placenta. Cells making up the ovum are beginning to differentiate and specialize in areas of development. Ovum has now implanted into the uterine wall
  • Week 6: Heart Begins to Beat

    Week 6: Heart Begins to Beat
    Embryo has reached a size of about 8mm, spinal cord is beginning to close, first blood cells and blood vessels are developing, pipe-shaped heart is formed and begins to beat, brain begins to divide into 5 parts, optic pits form the start of the developing eye, cells destined to be arms and legs are in place. Although not completely developed, all the major body organs and systems are formed
  • Week 7: Arms and Legs Begin to Grow

    Week 7: Arms and Legs Begin to Grow
    Embryo is around 13mm, heart is beating with one chamber, dividing wall is formed in the heart, arm and leg buds begin to grow, lower jaw and vocal cords begin to form, mouth opening is formed, inner ear is being created, digestive tract is developing, navel string being created, lungs, liver, pancreas, and thyroid gland are being formed
  • Week 8: Starts to Practice Moving

    Week 8: Starts to Practice Moving
    Embryo measures about 18mm in length, arm and legs are growing and location of elbows and toes are visible, feet and hand buds have appeared, starts to practice moving (not felt by mom until week 20), stomach is being made, face is beginning to take shape, baby's mouth and nostrils are starting to develop, teeth begin to develop, eyes can be seen as small hallows
  • Week 9: Fingers and Toes Form

    Week 9: Fingers and Toes Form
    Embryo has grown to 30mm, fingers and toes are well defined, cartilage and bones begin to form, upper lip and nose tip are being formed, tongue begins to develop, larynx is developing, eyelids are developed, main construction of heart is complete. During this week of pregnancy your baby is now swimming around in a little bag of fluid
  • Week 10: Embryo is Now a Fetus

    Week 10: Embryo is Now a Fetus
    The developing organism is now called a fetus, fetus is now the size of a small strawberry, feet are 2mm long, neck is beginning to take shape, body muscles are almost developed and baby has began moving, jaws are in place, mouth cavity and nose are joined, ears and nose can now be seen clearly, fingerprints are now evident in skin, nipples and hair follicles begin to form
  • Week 11: Neurons Multiply

    Week 11: Neurons Multiply
    Fetus CRL is about 1.5 inches in size, fingers and toes have completely seperated, taste buds are developing, baby has tooth buds (20 milk teeth), baby can swallow and stick out tongue, whole body except tongue is sensitive to touch, cartilage now calcifying to become bones, if boy- testicles produce testosterone hormone
  • Week 12: Fingernails and Toenails Appear

    Week 12: Fingernails and Toenails Appear
    Fetus is now 2.5 inches in length and weighs about 0.7 ounce, feet are almost half an inch long, fetus starts moving spontaneously, face is beginning to look like a baby's face, pancreas is functioning and producing insulin, fingernails and toenails appear, baby can suck thumb and get hiccups
  • Week 13: Baby Flexes and Kicks

    Week 13: Baby Flexes and Kicks
    Fetus has grown to about 3 inches long, weight is 1 ounce, bone is beginning to replace cartilage, ribs are appearing, nose and chin are well defined, movements can be measured, child can open/close mouth, external genitalia are almost defined- may be possible to tell gender
  • Week 14: Hormones Start Up

    Week 14: Hormones Start Up
    Fetus is 3.5 inches long, weight is 1.5 ounces, eyes are slowly moving towards center of face, nose is more defined, ears are fully developed, check bones are visible, first hair is appearing, kidneys are producing urine, in girls ovaries move down towards the pelvis, in boys the prostate gland is developing
  • Week 15: Bones and Muscles Grow

    Week 15: Bones and Muscles Grow
    Baby is now over 4 inches long, weighs about 2.5 ounces, he or she can move arms and make a fist, fingernails and toenails are present, baby's skin is still thin
  • Week 16: Facial Expressions are Possible

    Week 16: Facial Expressions are Possible
    Fetus measures nearly 5 inches, fetus weighs 3.5 ounces, fetus is covered with a protective soft down to regulate its temperature, fat begins to form underneath skin, baby hears external voices, sleeps and dreams
  • Week 17: Fat Accumulates

    Week 17: Fat Accumulates
    Umbilical cord is growing thicker and stronger, fetus weighs just over 5 ounces, baby is about 5 inches long, retina has become sensitive to light, first stools (meconium) are now beginning to accumulate
  • Week 18: Baby Begins to Hear

    Week 18: Baby Begins to Hear
    Fetus is now 6 inches long, weighs 7 ounces, can hear sounds and may be startled by loud noises, recognizable active and rest periods, skin is building a protective wax layer (vermix). Baby will have a favorite position for sleep
  • Week 19: Genitals Recognizable

    Week 19: Genitals Recognizable
    Fetus is around 6.5 inches and 9 ounces, placenta continues to grow and nourish the baby, baby's heart can be visible during ultrasound, baby's genitlas are distinct and recognizable, scalp hair has sprouted and continues to grow
  • Week 20: The Halfway Point

    Week 20: The Halfway Point
    Baby now weighs 11 ounces and is roughly 7 inches long, baby can hear and recognize mothers voice, mother will probably start feeling the fetal movements, toenails and fingernails are growing, growth of hair on rest of body has started, skin is getting thicker, heart can now be heard with stethoscope
  • Week 21: Nourishment Evolves

    Week 21: Nourishment Evolves
    Fetus has grins to 10.5 inches and weighs 12.7 ounces, now gaining weight steadily with fat being added to the body, bone marrow has started making blood cells, small intestine starting to absorb sugar
  • Week 22: Taste Buds Develop

    Week 22: Taste Buds Develop
    Baby is about 11 inches long and weighs about 1 pound, fetus reacts to loud sounds, baby starts having regular sleeping and waking rhythm, mothers movement can wake her baby, taste buds are forming and baby's tongue. Eyebrows and eyelids are fully developed
  • Week 23: Sense of Balance Develops

    Week 23: Sense of Balance Develops
    Baby is over 11 inches tall and weighs 1 pound, eyebrows are visible, child can successfully suck, bones located in middle of ear harden
  • Week 24: Practices Breathing

    Week 24: Practices Breathing
    Child is now gaining around 3 ounces a week, baby weighs about 1.3 pounds and is 12 inches long, eyelids can be seen very clearly, lungs are developing. Baby practices breathing by inhaling amniotic fluid into developing lungs
  • Week 25: Exploration Continues

    Week 25: Exploration Continues
    Baby is around 13.5 inches in length and weighs 1.5 pounds, bones are becoming solid and hands are fully developed, brain is growing rapidly and brain cells are starting to mature, sexual organs are fully developed
  • Week 26: Second Trinester Ends

    Week 26: Second Trinester Ends
    Baby measures 14 inches long and weighs over 1.7 pounds, feet are 2 inches long and hands are very active, baby's eyes begin to open around this time
  • Week 27: Eyes Can Open

    Week 27: Eyes Can Open
    Fetus weighs almost 2 pounds and is about 14.5 inches long, head is over 2.5 inches in diameter, baby's hearing continues to develop, eyes can blink open and close
  • Week 28: Recognize Your Voice

    Week 28: Recognize Your Voice
    Fetus has its own interval for sleeping and being awake, 2.2 pounds weight and is 14.8 inches in length, feet are just over 2 inches long, hair on head is clearly visible, milk teeth have developed under gums, eyes are starting to move in sockets
  • Week 29: Movement is More Forceful

    Week 29: Movement is More Forceful
    Baby now weighs 2.5 pounds and is about 15 inches long, brain is busy developing billions of neurons, baby's muscles and lungs are continuing to mature, fetus can taste and respond to pain, milk teeth have developed under gums
  • Week 30: Baby Puts on Pounds

    Week 30: Baby Puts on Pounds
    Baby measurements are 16 inches and weighs about 3 pounds, head is almost 3.5 inches in diameter, feet are nearly 2.5 inches long, wrinkled skin is now becoming smoother, in boys-testicles have moved down towards growin, baby controls its own body temperature
  • Week 31: Baby Detects Light

    Week 31: Baby Detects Light
    Baby weighs 3.5 pounds and measures 16.5 inches, eyes have now completely opened and are responding to light and darkness, a loud noise near you may cause baby to jump
  • Week 32: Reproductive Development Continues

    Week 32: Reproductive Development Continues
    Baby is 17 inches long and weighs 4 pounds, diameter of head is almost 4 inches, under the skin the fat layer is getting thicker, toenails and fingernails are completely formed, in boys-the testicles will be descending from the groin down into the scrotum
  • Week 33: Lanugo is Disappearing

    Week 33: Lanugo is Disappearing
    Baby is about 4.5 pounds and 17.5 inches in length, baby fuzz lanugo is disappearing, baby drinks about a pint of amniotic fluid a day, half of your weight gain will go to your baby
  • Week 34: Rapid Growth Continues

    Baby is now 18 inches long and weighs 5.25 pounds, child now has excellent chance of survival outside of whom, fat accumulation plumps up the arms and legs this week, eyes open when awake and close when asleep, fingernails are now completely formed
  • Week 35: Reflexes are Coordinated

    Week 35: Reflexes are Coordinated
    Baby is now 18.5 inches inleght and weighs over 5.5 pounds, body of baby is growing round due to developing fat layers, baby's reflexes are coordinated, lungs are almost fully developed
  • Week 36: Baby May drop into Birth Canal

    Week 36: Baby May drop into Birth Canal
    Baby is about 19 inches long and weighs around 6 pounds, baby's body is becoming chubby as fat layer builds, between now and birth they may gain an ounce a day
  • Week 37: Pregnancy is Considered at Term

    Week 37: Pregnancy is Considered at Term
    Typically baby now measure 19.5 inches and weighs up to 7 pounds, head diameter is over 3.5 inches, water could break at any time
  • Week 38: Development is Complete

    Week 38: Development is Complete
    Baby is about 20 inches and weighs about 7.5 pounds, the soft down which has covered the baby through the pregnancy is now disappearing, body fat is continuing to build up and baby is putting on 1 ounce a day, wrinkled skin is becoming baby smooth, baby may have reached final birth position
  • Week 39: Very Close Now

    Average baby is about 20 inches and weighs 7.5 pounds, baby has reached final birth position, it will be cramped inside the uterus for much movement, baby will continue to punch and kick but lower in your abdomen under your pelvis, head is about 4 inches in diameter
  • Week 40: Due date Arrives

    Now reached time of delivery, all system are developed, baby is 20 inches and weighs approximately 7.5 pounds, head has a diameter of about 4 inches, contractions can begin at any moment