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Week 7

First movements and responses to touch

Human Prenatal Age

Post-conception week 5
Days of life 35-41
Gestational Week 7

Highlights

The unborn baby starts moving and responds to a light touch near his mouth.¹
Nearly every major organ — from brain to heart— is already taking shape in a remarkable display of biological engineering.
Eye pigments start forming this week,² but the baby’s eye color will not finalize until about 10 months after birth.³

At 7 weeks of pregnancy, the unborn baby is about 10,000 times bigger than he was at conception. That makes him the size of a blueberry, or a little under a half inch from rump to head. This week, the unborn baby starts moving, and the first sense develops: touch.⁴ Prenatal movement promotes normal bone and neuromuscular development.⁵

First movements and brain development

By 7 ½ weeks gestation, the baby responds to a thin paintbrush tickling his lips or cheeks by reflexively moving away!⁶ ⁷ Before the unborn baby can move, connections must form between his neurons and muscles. The first connections between neurons and muscles are observed just before the first fetal movements.⁸ The first movements start slow and simple, with a sideways bending of the head and body,⁹ ¹⁰ ¹¹ ¹² ¹³ ¹⁴ and quick startle responses¹⁵ but grow more complex in subsequent weeks. These movements also start off fairly rare – at 7 weeks only 18 to 29 seconds of movement were seen in one hour of continuous ultrasound recordings.¹⁶ As the baby grows, the movements get more complex and frequent until by 15 weeks the baby usually moves every two minutes for about a minute and a half at a time!¹⁷

In the first few weeks of the baby’s life, brain growth is astonishing! At week 7, the brain starts producing neurons at the incredible pace of 250,000 neurons per minute. ¹⁸ ¹⁹ This is especially shocking given that the brain essentially stops creating new neurons about 2 months before birth.²⁰ ²¹

By the end of week 7, the brain has separate right and left cerebral hemispheres.²² The cerebral hemispheres will direct speech, decision making, movement, balance, vision, memory, and many other functions. Seven weeks gestation is also the first time that substance P and enkephalins, two signaling molecules in the nervous system, also known as neurotransmitters, dedicated to processing pain, have been detected.²³

This composite image rendered from light-guided endoscopy recordings shows the embryo alive in the uterus at 5 weeks after [tooltip anchor="fertilization"]Sperm-egg fusion[/tooltip] (or 7 weeks gestation). Notice how the skin is still mostly transparent and tiny hands and feet are beginning to take shape. (Image Credit: <a href="https://erf.science/#high-resolution">Education Resource Fund</a>)

This composite image rendered from light-guided endoscopy recordings shows the embryo alive in the uterus at 5 weeks after (or 7 weeks gestation). Notice how the skin is still mostly transparent and tiny hands and feet are beginning to take shape. (Image Credit: Education Resource Fund)

By 7 ½ weeks gestation, the baby moves in response to a light touch near his mouth.²⁴

The human heart at 7 weeks of pregnancy. (Image credit: Science Source)

Heart development at 7 weeks gestation

As the week progresses, the heart continues to fold and loop, curling into an S-shape. This folding is crucial because it helps form the four chambers that make up the mature heart. Blood now circulates more efficiently, supplying even more oxygen and nutrients to the growing and developing body.²⁵

Furthermore, the heart starts dividing into right and left sides. Thin walls, or septa, grow to separate the chambers, while small openings allow for temporary blood flow between them—an adaptation perfectly suited for life in the womb, where oxygenated blood comes from the placenta. Early heart valves are forming.²⁶

The heart’s pacemaker cells also continue to develop in week seven. These cells integrate into the right atrium of the heart.²⁷ From this position, these pacemaker cells work in cooperation with nerve impulses to control a person’s heart rate for the rest of his life.

When does the placenta start working?

The shift to placental nutrition happens gradually. Before 7 weeks of gestation, the baby relies mainly on the yolk sac and uterine lining for nourishment while the placenta continues to form.²⁸ By week 7, the placenta and umbilical cord are developed enough to take over the primary role of delivering oxygen and nutrients to the unborn baby and removing wastes like carbon dioxide.²⁹ By 10 weeks, the placenta is the primary source of life, providing everything the unborn baby needs to survive.³⁰ Before the placenta is mature, the baby has less oxygen. Fetal blood can carry roughly 40% more oxygen than maternal blood, allowing the baby to thrive in low-oxygen environments.³¹ Overall, the placenta’s life-support capabilities create an irreplacable bond between the mother and child.

The face at 7 weeks of pregnancy

In gestational week 7, the oropharyngeal membrane, a membrane that closed off the digestive tract, disintegrates. Now the amniotic fluid can enter the unborn baby’s mouth and digestive system. Furthermore, muscle cells start growing to control facial expressions and jaw movements.³² External ears also appear, and the eyes begin to move toward the front of the face as the baby’s head grows and develops.³³

This baby's tongue formed from three swellings that fused together while he was still and embryo. By 34 weeks gestation, he smiled more when the amniotic fluid tasted sweet. (Image Credit: Adobe Stock Photos)

How does the tongue form?

Tongue formation is a true marvel of nature. The tongue develops from pharyngeal arches, which are paired bulges on either side of the baby’s neck. In week 6, a tiny tissue swelling appears in the area where the tongue will grow. In week 7, two more swellings start to grow on either side. These three swellings all come from the first pharyngeal arch and merge to form the frontal two-thirds of the tongue.³⁴ The back of the tongue develops from swellings in the second, third, and fourth pharyngeal arches. The muscles in the tongue come from early muscle fibers called myoblasts. There are four muscles which allow the tongue to move in all directions, and another four pairs of muscles are responsible for changing the shape of the tongue.³⁵ These specialized muscles allow the tongue to perform many complex tasks, such as chewing, swallowing, and speaking. By week 9, taste buds begin to develop, as do the nerve fibers that connect the taste buds to the brain,³⁶ preparing the unborn child to savor lots of flavors. By 34 weeks, the unborn baby will make happier faces after the mom eats something sweet than bitter.³⁷ The tongue both grows and functions in an incredibly precise way.

Eggs and Sperm

Between 300 and 1300 primordial germ cells migrate from the yolk sac to the developing gonad which will become the ovaries or testes.³⁸ These germ cells will differentiate into early sperm cells called spermatogonia in males and early egg cells called oogonia in females.³⁹ In a woman’s body, oogonia give rise to primary oocytes or “eggs”, which remain dormant in the ovaries for up to 40 years. Interestingly, no primary oocytes form after birth; a woman’s body creates all of her eggs starting at seven weeks gestation and finishing around five months.⁴⁰ Similarly, in a man’s body, the primary germ cells that migrate in week 7 become spermatagonia, which will create all the man’s sperm.⁴¹ Interestingly, a single oogonia becomes one egg while a single spermatagonium becomes 4 sperm.⁴²

How does the bladder form?

Bladder development begins around week 7. The bladder first forms as a ‘cloaca’, a common chamber for the early kidneys and gut. By 10 weeks, the cloaca has divided into two tubes; one becoming the end of the intestines, and one becoming the urethra to help empty the bladder.⁴³ ⁴⁴ As early as 9 weeks, ultrasound has confirmed the presence of urine within the bladder, showing functional kidney activity.⁴⁵ By 14 weeks, subtle anatomical differences emerge between male and female babies, with changes in the position of the bladder.⁴⁶ Interestingly, in both infants and young children, the bladder remains located within the abdominal cavity. It does not descend into the pelvis until around six years old, ultimately reaching its adult position during puberty.⁴⁷

Continued development at 7 weeks of pregnancy

At 7 weeks gestation, in the second month, the unborn baby starts receiving nutrients and oxygen and expelling wastes like carbon dioxide using the umbilical cord and placenta.⁴⁸

The limb buds keep lengthening, with the arm buds growing one or two days before the leg buds. Nerves enter the limb buds.⁴⁹ Tiny hands and feet begin to take shape as the ends of the buds start flattening into paddle shapes called hand and foot plates.⁵⁰

At 5 weeks, the baby’s skin was only one cell thick. Now, a second layer is forming.⁵¹ The skin is still very thin and transparent. It can release water and salts into the amniotic fluid.⁵²

Eye pigments begin forming this week,⁵³ but a baby’s final eye color may not be set until around 10 months after birth.⁵⁴

A chart titled Pregnancy Timeline shows pregnancy months with their corresponding weeks: Month 1 (weeks 1-4), Month 2 (5-8), Month 3 (9-13), Month 4 (14-17), Month 5 (18-22), Month 6 (23-27), Month 7 (28-31), Month 8 (32-35), Month 9 (36-40).

Dive Deeper

Future Eggs and Sperm

Did you know that all of a woman’s eggs formed before she was born?

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