6 weeks pregnant: Changes for baby

How big is your baby now?

The beginning of week 6 of pregnancy is the fourth week of your baby’s development. Your baby is about 2mm in length (or about the size of a metal pinhead). It will double in size and grow to about 4mm (roughly the width of a USB plug) by the end of this week.

How is your baby growing?

Your baby is now called an embryo. Embryonic development, also referred to as embryogenesis, occurs from weeks four to eight of your babies growth (pregnancy weeks six to ten), before the foetal development period begins in pregnancy week 11. Embryonic development is the period in which formation of body organs (organogenesis) occurs and by the end of which your baby has taken on a distinctly human form. By the end of embryonic development and organogenesis (10 weeks pregnant or 8 weeks of your baby’s growth) each of the major organ systems of the body have begun their development.

During this week, many of the major organs start to form from the three germ layers (mesoderm, endoderm and ectoderm) which formed in pregnancy week 5. These three types of cells give rise to cells which will form each of the organs and tissues that comprise the human body- they include all the genetic information needed to grow ears and eyes, legs and a liver and all the other tiny parts which will form your baby’s body.

How is your baby’s appearance changing?

Formation of the foetus-like body shape

During the sixth week of pregnancy, your embryo changes from from a flat three layered disc into a curved foetus-like body shape. Folding of the flat disc occurs because the spinal cord and somites (the cells which will form the vertebrae) grow rapidly and cause the disc to lengthen too quickly. This causes the sides of the flat disc to curve inward and the embryo to take on a cylindrical shape in the early days of this week (day 21-22 of your baby’s growth). Folding of the disc into a cylinder produces two folds of tissue, one at each side of the embryonic body, called the right and left lateral folds.

At the same time that the sides are curving inwards to form a cylinder, the embryo’s top and bottom curve into a c shape, stimulated by rapid growth at the head and tail ends of the cylinder. The embryo curves its head and tail regions to form the c shape typical of a foetus. The front of the body forms the inner curve of the c and the back of the body forms the outer curve of the c. This curving causes your baby’s face and chest to move to the correct placements and allows identification of its head and tail ends, left and right, as well as its front and back sides. It also causes rearrangement of the cells forming the beginnings of baby’s internal organs (discussed below).

Distinct head shape

At the beginning of this week, the head end of the body is an open end of the developing neural tube (which will become the spinal cord) called the rostral neuropore. The neural tube begins to close in both directions from the baby’s middle, like a zipper zipping up in both directions. The rostral neuropore at the head end of baby’s body is the first to close and is fully ‘zipped-up’ by day 26 of your baby’s growth (day 5 of pregnancy week 6). 

The beginnings of your baby’s brain begin to form from folds of tissue in the head region, causing an oval-shaped protrusion at the head end of baby’s body. The prominent head shape of your baby is now visible and takes its structure from the growing brain. As the body curves into a c shape, the head moves to overhang a protrusion from the hollow region of the c, in which baby’s heart will grow.

Tail-like structure

The tail end of baby’s body, which at the beginning of the week is an open groove of the forming neural tube called the caudal neuropore, also zips up this week. By the end of the sixth week of pregnancy it is a closed structure called the caudal eminence which forms the tail of baby’s neural tube. It grows out over a structure called the cloacal membrane, which will eventually form your baby’s anus.

Continued development of the somites
The somites (beginning of your baby’s vertebrae), of which there were 4-12, protruding from baby’s body at the beginning of this week continue to grow. Counting the number of somites on an ultrasound is one way of determining your baby’s age in this early period of embryonic growth. At the end of this week there are approximately 30 pairs of somites (compared to a single pair at the end of 4 weeks of pregnancy).

Towards the end of this week, each somite differentiates into two parts:

• Schlerotome: which provides the basis for the growth of vertebrae and ribs; and
• Dermomyotyme including:
  • Myotome cells which provide the basis for the growth of muscles; and
  • Dermatome cells which provide the basis for the growth of fibroblast (supporting tissue).

Development of facial features- jaws, ears and eyes

Your baby’s face also begins to take on a more human form in week 6 of pregnancy. The structures which will grow into the jaws, ears and eyes of their tiny face begin to form. Two bulges, called the pharyngeal arches are visible by the third day of pregnancy week 6. By day 5 of this week, a third pair has developed. A fourth pair of arches have formed by the end of this week and pharyngeal arches five and six form by the end of pregnancy week 10.

The first pharyngeal arch will form your baby’s jaw bones. The second pair will form the cartilage and bone of baby’s ear. Along with the third pair of pharyngeal arches it will also form the bone of the upper neck called the hyloid bone. The fourth and sixth arches will eventually form the cartilage which surrounds baby’s larynx. The fifth pair of arches, which are not always visible, disappear as your pregnancy progresses.

Two thickened patches of endoderm tissue are also visible in the head region. Called the lens placode, these are the beginnings of the lens of your baby’s eyes. Otic pits, which will form the inner chamber of your baby’s ears, are also visible on the head by day 26 of embryo development (day 5 of the sixth week of pregnancy).

Limb buds

Upper and lower limb buds, which later will develop into your baby’s arms and legs also appear in week 6. The upper limb buds typically become visible on day 26 of your baby’s growth, the lower limbs buds appear one day later.

How are your baby’s internal organs developing?

Rearrangement and development of primordial (initial) organs

In week 6 of pregnancy the primordial organs (the organs in their first stage of development) rearrange themselves to keep pace with growth and curving of the embryo body. As the embryo body curves, a number of organs move towards the front of baby’s body and protrude into the hollow of the c curve. They include the:

• Septum traversum which will grow into the diaphragm (the diaphragm is a thin structure separating the abdominal and chest cavities which has a role in breathing);
• Primordial heart which will grow into the cardiovascular system; and
• Embryonic coelom which will grow to form internal body cavities like the stomach and colon (bowel).

Respiratory and gastrointestinal organs

In week 6 of pregnancy the foregut, which will eventually grow into organs of your baby’s respiratory system and digestive (gastrointestinal) system develops. Organs of the gastrointestinal system which are derived from the foregut include the stomach and duodenum (beginning of the small intestine), liver, pancreas (a hormone-producing gland) and gall bladder and associated bile-producing organs. The respiratory organs which eventually form from the foregut include the pharynx (tube connecting nose and mouth), oesophagus (windpipe) and lower respiratory tract (tubes connecting to the lungs).

As the embryo folds its head and tail regions, part of the umbilical cord moves into the embryo’s body to develop as the foregut. It lies in the hollow of the c-shaped curve, between the developing brain located at the head of your baby’s body and a bulge called the primordial heart. Beside the foregut, but separated from it by a structure called the oropharyngeal membrane is the stomodeum, the structure which will eventually form your baby’s mouth.

A group of cells from the endoderm layer which will grow into the small intestine also moves into the embryo in week 6 of pregnancy. Known as the midgut (which will grow into baby’s small intestine), these cells are connected to the umbilical vesicle (the structure which transfers nutrients to the embryo). The initially wide connection between the midgut and umbilical vesicle narrows into the beginnings of the relatively thin umbilical cord (the cord which attaches the placenta and the foetus), as the embryo body curves.

The hindgut (lower intestine/colon/bowel) also grows from a group of endoderm cells which are incorporated into the embryo body in its fourth week of development (week 6 of pregnancy). The hindgut includes a structure called the cloaca which will grow into your baby’s bladder and rectum.

Cardiovascular organs

The beginnings of your baby’s cardiovascular system, the primordial heart, form a prominent bulge in the chest region by the 3^rd day of this week. At the same time it begins pumping blood through the embryo. In short, your baby now has a heartbeat!

Continued formation of neural structures

The neural tube which began to form as an open groove running the length of your baby’s body opposite the somites last week, continues to ‘zip-up’ from the mid section in both directions this week. The head end of the neural tube, called the rostral neuropore closes first (on the 26^th day after conception, which is usually day 5 of pregnancy week 6). It zips up around two folds of tissue which will grow into your baby’s brain.

The tail end of baby’s body, called the caudal neuropore, closes a day later when the neural tube (beginnings of the spinal cord) becomes fully formed between the pairs of somites which will form vertebrae.

Brain

On day 22 of embryo development (the second day of pregnancy week 6) the two neural folds in the head region begin to thicken and form your baby’s brain. At the beginning of the 6^th pregnancy week when the embryo remains flat, the developing brain grows in the back of the head and extends into the amniotic cavity. Later in the week when the body curves, the brain moves to the front of the head and overhangs a bulge at the front of the embryo body which will form the heart. The growing brain gives the head shape to the embryo body.

Development of supporting structures-umbilical cord and amniotic and chorionic sacs

Umbilical cord

The connecting stalk which will go on to become the umbilical cord attaches to the front of your baby’s body towards the end of this week.

Amniotic and chorionic sacs

The amniotic sac (the sac which surrounds the embryo and contains amniotic fluid which protects baby in the womb and allows it to move around) and chorionic sac (which will grow into part of the placenta), previously attached to the embryo body, move away in the first half of this week leaving the embryo exposed.

Features that help determine the age of the embryo

By the end of this week there are several external characteristics that can be used to determine your baby’s age using an ultrasound. They include-

• Body shape: the embryo body curves into a c-shape this week;
• Somites: approximately 30 pairs of somites (which will grow into baby’s vertebrae) have developed and run down the length of your baby’s body;
• Distinct head appears;
• Buds which will grow into the arms and legs begin to develop;
• The otic pits, which later go on to form the internal ears are visible;
• The lens placodes, the future lens of the eyes, are visible; and
• A long tail-like structure is visible.

References

For more information about week 6 of pregnancy

For more information about the sixth week of pregnancy see 6 weeks pregnant

For more details about the changes for mum in week 6 of pregnancy see 6 weeks pregnant: Changes for mum

For more detailed information about pregnancy health in the sixth week of pregnancy see 6 weeks pregnant: Doctor appointments and health information

For a summary of the key point about the sixth week of pregnancy see 6 weeks pregnant- Key points