embryos development timeline.

Week 1–2 of gestational age are theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.

Fertilization of the ovum to form a new human organism, the human zygote. (day 1 of fert.) The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5–3 of fertilization)

Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7–12 of fertilization.

A notochord forms in the center of the embryonic disk.Gastrulation commences.

The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appears.
Branchial arches, grooves which will form structures of the face and neck, form.
The neural tube closes.
The ears begin to form as otic pits.
Arm buds and a tail are visible.

Lens pits and optic cups form the start of the developing eye.
Nasal pits form.
The brain divides into 5 vesicles, including the early telencephalon.
Leg buds form and hands form as flat paddles on the arms.
Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.
The metanephros, precursor of the definitive kidney, starts to develop.
The initial stomach differentiation begins.

The embryo measures 13 mm (1/2 inch) in length.
Lungs begin to form.
The brain continues to develop.
Arms and legs have lengthened with foot and hand areas distinguishable.
The hands and feet have digits, but may still be webbed.
The gonadal ridge begins to be perceptible.
The lymphatic system begins to develop.
Main development of external genitalia starts.

Your new resident is nearly an inch long — about the size of a grape — and weighs just a fraction of an ounce. She's starting to look more and more human. Her essential body parts are accounted for, though they'll go through plenty of fine-tuning in the coming months. Other changes abound: Your baby's heart finishes dividing into four chambers, and the valves start to form — as do her tiny teeth. The embryonic "tail" is completely gone. Your baby's organs, muscles, and nerves are kicking into ge

Though he's barely the size of a kumquat — a little over an inch or so long, crown to bottom — and weighs less than a quarter of an ounce, your baby has now completed the most critical portion of his development. This is the beginning of the so-called fetal period, a time when the tissues and organs in his body rapidly grow and mature.
He's swallowing fluid and kicking up a storm. Vital organs — including his kidneys, intestines, brain, and liver (now making red blood cells in place of the disa

Your baby, just over 1 1/2 inches long and about the size of a fig, is now almost fully formed. Her hands will soon open and close into fists, tiny tooth buds are beginning to appear under her gums, and some of her bones are beginning to harden.
She's already busy kicking and stretching, and her tiny movements are so effortless they look like water ballet. These movements will become more frequent as her body grows and becomes more developed and functional. You won't feel your baby's acrobatics

The most dramatic development this week: reflexes. Your baby's fingers will soon begin to open and close, his toes will curl, his eye muscles will clench, and his mouth will make sucking movements. In fact, if you prod your abdomen, your baby will squirm in response, although you won't be able to feel it. His intestines, which have grown so fast that they protrude into the umbilical cord, will start to move into his abdominal cavity about now, and his kidneys will begin excreting urine into his

Fingerprints have formed on your baby's tiny fingertips, her veins and organs are clearly visible through her still-thin skin, and her body is starting to catch up with her head — which makes up just a third of her body size now. If you're having a girl, she now has more than 2 million eggs in her ovaries. Your baby is almost 3 inches long (the size of a medium shrimp) and weighs nearly an ounce.

This week's big developments: Your baby can now squint, frown, grimace, pee, and possibly suck his thumb! Thanks to brain impulses, his facial muscles are getting a workout as his tiny features form one expression after another. His kidneys are producing urine, which he releases into the amniotic fluid around him — a process he'll keep up until birth. He can grasp, too, and if you're having an ultrasound now, you may even catch him sucking his thumb.
In other news: Your baby's stretching out. F

Your growing baby now measures about 4 inches long, crown to rump, and weighs in at about 2 1/2 ounces (about the size of an apple). She's busy moving amniotic fluid through her nose and upper respiratory tract, which helps the primitive air sacs in her lungs begin to develop. Her legs are growing longer than her arms now, and she can move all of her joints and limbs. Although her eyelids are still fused shut, she can sense light. If you shine a flashlight at your tummy, for instance, she's like

Get ready for a growth spurt. In the next few weeks, your baby will double his weight and add inches to his length. Right now, he's about the size of an avocado: 4 1/2 inches long (head to rump) and 3 1/2 ounces. His legs are much more developed, his head is more erect than it has been, and his eyes have moved closer to the front of his head. His ears are close to their final position, too. The patterning of his scalp has begun, though his locks aren't recognizable yet. He's even started growing

Your baby's skeleton is changing from soft cartilage to bone, and the umbilical cord — her lifeline to the placenta — is growing stronger and thicker. Your baby weighs 5 ounces now (about as much as a turnip), and she's around 5 inches long from head to bottom. She can move her joints, and her sweat glands are starting to develop.

Head to rump, your baby is about 5 1/2 inches long (about the length of a bell pepper) and he weighs almost 7 ounces. He's busy flexing his arms and legs — movements that you'll start noticing more and more in the weeks ahead. His blood vessels are visible through his thin skin, and his ears are now in their final position, although they're still standing out from his head a bit. A protective covering of myelin is beginning to form around his nerves, a process that will continue for a year after

Your baby's sensory development is exploding! Her brain is designating specialized areas for smell, taste, hearing, vision, and touch. Some research suggests that she may be able to hear your voice now, so don't be shy about reading aloud, talking to her, or singing a happy tune if the mood strikes you.
Your baby weighs about 8 1/2 ounces and measures 6 inches, head to bottom — about the size of a large heirloom tomato. Her arms and legs are in the right proportions to each other and the rest o

Your baby weighs about 10 1/2 ounces now. He's also around 6 1/2 inches long from head to bottom and about 10 inches from head to heel — the length of a banana. (For the first 20 weeks, when a baby's legs are curled up against his torso and hard to measure, measurements are taken from the top of his head to his bottom — the "crown to rump" measurement. After 20 weeks, he's measured from head to toe.)
He's swallowing more these days, which is good practice for his digestive system. He's also pro

Your baby now weighs about three-quarters of a pound and is approximately 10 1/2 inches long — the length of a carrot. You may soon feel like she's practicing martial arts as her initial fluttering movements turn into full-fledged kicks and nudges. You may also discover a pattern to her activity as you get to know her better. In other developments, your baby's eyebrows and lids are present now, and if you're having a girl, her vagina has begun to form as well.

At 11 inches (the length of a spaghetti squash) and almost 1 pound, your baby is starting to look like a miniature newborn. His lips, eyelids, and eyebrows are becoming more distinct, and he's even developing tiny tooth buds beneath his gums. His eyes have formed, but his irises (the colored part of the eye) still lack pigment.

Turn on the radio and sway to the music. With her sense of movement well developed by now, your baby can feel you dance. And now that she's more than 11 inches long and weighs just over a pound (about as much as a large mango), you may be able to see her squirm underneath your clothes. Blood vessels in her lungs are developing to prepare for breathing, and the sounds that your baby's increasingly keen ears pick up are preparing her for entry into the outside world. Loud noises that become famili

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