Table of Contents
Page created on August 18, 2020. Not updated since.
Learning objectives
- At which point does the embryo become a foetus?
- Which changes to the external appearence occurs during the third week?
- Which changes to the external appearance occurs during the second month?
- What is the crown-rump length?
- What is the difference between gestational age and conceptional age?
- When does birth usually occur?
- How is the gestational age of a foetus determined?
- How does the head develop in comparison to the body?
- Which changes to the external appearance occurs during the third month?
- Which changes to the external appearance occurs during the fourth and fifth months?
- Which changes to the external appearance occurs during the sixth month?
- Which changes to the external appearance occurs during the last months?
- What are the signs of maturity in the newborn?
- What is the most common cause of abnormally high birth weight?
- What is the definition of a premature infant?
- What is the definition of a postmature infant?
- Why is postmaturity dangerous?
External appearance of embryo
The newly developing human is an embryo from the point of fertilization until the end of week 8. From week 9 and onwards the foetal stage begins, and so the newly developing human is then known as a foetus.
The period of organogenesis occurs during weeks 3 – 8. All major organs develop in this period.
Development during week 3
Day 20 – the embryo is a trilaminar disc.
Day 22 and 23 – closure of the neural tube begins and finishes.
Day 28 – the somites, pharyngeal arches, heart, and liver primordia form.
Development during the second month
During weeks 4 and 5 the limbs are paddle-shaped, but they flatten during week 6. At the ends of the limbs digital rays appear, which will give rise to the fingers. Apoptosis (programmed cell death) will occur between the digits. This separates the fingers from each other.
During week 6 the physiological herniation of the intestines occurs, causing the umbilical cord to be swollen proximally.
During week 7 the limbs are segmented into two portions (divided by the elbow and knee)
External appearance of foetus
Crown-rump length
The crown-rump length, often abbreviated as CRL, is the length of the embryo or foetus as measured by the distance from the head to the coccyx or the bottom of the torso. Another measurement, the crown-heel length (CHL), is less frequently used.
During embryonic development, the length and weight of the embryo and foetus increase rapidly. Length increases especially during the third, fourth, and fifth months, while weight increases especially during months 8 and 9. The table below shows how the CRL and weight changes with the gestational age. You’re not required to know this table, but you should have an approximate idea of the size of the embryo and foetus at different ages, and at birth.
|
Gestational age (week) |
Crown-rump length (cm) | Weight (grams) |
| 9 – 12 | 5 – 8 |
10 – 45 |
|
13 – 16 |
9 – 14 | 60 – 200 |
| 17 – 20 | 15 – 19 |
250 – 450 |
|
21 – 24 |
20 – 23 | 500 – 820 |
| 25 – 28 | 24 – 27 |
900 – 1300 |
|
29 – 32 |
28 – 30 | 1400 – 2100 |
| 33 – 36 | 31 – 34 |
2200 – 2900 |
|
37 – 38 |
35 – 36 |
3000 – 3400 |
Taken from Langman’s Medical Embryology, p. 106
Gestational age vs conceptional age
Birth occurs at approximately 266 days (38 weeks) after fertilization; however, the exact date of fertilization is very difficult to determine. This is the conceptional age. Due to the difficulty in determining it accurately, it’s rarely used in medical practice. It may be used during research.
The gestational age is easier to work with, so we use that instead. The gestational age is counted from the first day of the last normal menstrual period. Birth occurs at approximately 280 days (40 weeks) after the first day of the last normal menstrual period.
Determination of gestational age
The CRL is pretty characteristic for the gestational age, which makes it useful in determining the gestational age. CRL measurement by ultrasound is used to estimate gestational age and expected day of delivery, but only during weeks 7 – 14.
During weeks 16 to 30, gestational age is estimated by other measures. These measures include the femoral length, abdominal circumference, and something called the biparietal diameter (BPD). The BPD is the diameter of the head when measured between the parietal prominences.
Large head
A notable change during embryonic development is how the head initially grows faster than the rest of the body. At the beginning of the third month, the head accounts for approximately half of the CRL. This is reduced to one-third at the beginning of month 5, and to one-fourth at birth.
Development during the third month
During the third month the foetus becomes more human-looking. This is mostly due to the eyes moving from the lateral aspect to the face to the ventral aspect, and the ears migrating from the neck to their definitive position.
By the 12th week the external genitalia of the foetus are so developed that the sex of the foetus can be determined by ultrasound. At the same time, the intestinal loops have retracted into the abdominal cavity, so the umbilical cord is no longer swollen.
At this point the limbs are proportional to the body.
Development during the fourth and fifth months
During these months the foetus lengthens considerably, but its weight doesn’t increase much. The foetus is covered with fine hair called lanugo hair, and the eyebrows and head hair are also present.
The mother starts to feel the movements of the foetus during the fifth month.
Development during the sixth month
At this point the skin of the foetus is very thin due to the lack of underlying connective tissue and subcutaneous fat. This causes the skin to be reddish and wrinkly.
Sebaceous glands on the skin of the foetus produce a whitish, fatty substance called vernix caseosa. This substance is theorized to moisturize and protect the infant’s skin and facilitate passage through the birth canal.
Development during the last months
After week 28 the lungs start to produce significant amounts of surfactant. Development of subcutaneous connective tissue and fat causes the skin to be less wrinkly and thicker.
At the end of the ninth month, the skull has the largest circumference of all parts of the body. This is important with regard to its passage through the birth canal.
Signs of maturity
- Length 50 cm ± 10
- Weight 3 kg ± 500 g
- Birth weight > 3500 g is related to maternal diabetes
- Head circumference ~35 cm
- Descended testicles/labia minora covered by labia majora
- Well-formed ears, with normal stiffness
- Open eyes
- Plantar creases present on the foot
- Nails cover the fingertips
- Reflexes are present
- Indicate mature CNS
- Absence of lanugo hair
- Some healthy full-term newborns have lanugo hairs at birth
Prematurity
An infant is premature if it’s born at less than 37 weeks of gestation. Premature infants have higher risk for severe complications, especially neonatal respiratory distress syndrome and cardiovascular disease, and this risk increases the lower the gestational age. There are several external signs of prematurity:
- Low birth weight
- Small size
- Weak or absent crying
- Low muscle tone
- Undescended testes/underdeveloped female genital organs
- Soft ears
- Thin, smooth, pinkish skin
- Presence of lanugo hair
Postmaturity
An infant is postmature if it’s born at more than 42 weeks of gestation. This is rare but carry risks for both the mother and the infant. After week 42 the placenta begins to fail. Labour is usually induced medically before this point.
Summary
- At which point does the embryo become a foetus?
- From week 9 and onwards
- Which changes to the external appearence occurs during the third week?
- At day 20 the embryo is a trilaminar disc
- At day 22 the neural tube begins to close
- At day 28 the somites, pharyngeal arches, heart, and liver primordia form
- Which changes to the external appearance occurs during the second month?
- During week 6 the limbs flatten. Digital rays form at the ends
- Physiological herniation also occurs in week 6, causing the umbilical cord to be swollen proximally
- During week 7 the limbs are segmented into two portions
- What is the crown-rump length?
- The length from the top of the head to the bottom of the torso
- What is the difference between gestational age and conceptional age?
- Conceptional age is the age of the foetus from the time of fertilization
- Gestational age is the age of the foetus from the first day of the last normal menstrual period
- When does birth usually occur?
- At 280 days (40 weeks) of gestation
- How is the gestational age of a foetus determined?
- CRL is used to estimate gestational age between weeks 7 – 14
- Biparietal diameter, femoral length, and abdominal circumference are used during weeks 16 – 30
- How does the head develop in comparison to the body?
- The head initially grows much faster than the body, causing the head to account for half of the CRL at the beginning of the third month
- As time goes on the growth of the head slows down, causing the head to account for one-fourth of the CRL at birth
- Which changes to the external appearance occurs during the third month?
- The eyes move from the lateral aspect to the ventral aspect, and the ears move to their definitive position
- The external genitalia can be visualized by ultrasound
- The intestines retract into the abdominal cavity
- The limbs become proportional to the body
- Which changes to the external appearance occurs during the fourth and fifth months?
- The foetus lengthens considerably
- Lanugo hair, eyebrows, and head hair develop
- Which changes to the external appearance occurs during the sixth month?
- Sebaceous glands produce vernix caseosa
- Which changes to the external appearance occurs during the last months?
- Subcutaneous connective tissue and fat causes the skin to be less wrinkly and thicker
- Lungs begin to produce surfactant at week 28
- What are the signs of maturity in the newborn?
- See above
- What is the most common cause of abnormally high birth weight?
- Maternal diabetes
- What is the definition of a premature infant?
- Born at earlier than 37 weeks of gestation
- What is the definition of a postmature infant?
- Born at later than 42 weeks of gestation
- Why is postmaturity dangerous?
- The placenta begins to fail at 42 weeks of gestation