Growth & Development of the Fetus
- After fertilisation in the oviduct, the zygote travels towards the uterus
- This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
- In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop
- The gestation period for humans is 9 months
- Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining
- After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus
- The remaining gestation time is used by the fetus to grow bigger in size
The fetus in the uterus
- The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s blood plasma)
- This protects the fetus during development by cushioning it from bumps to the mother’s abdomen
- The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products
The Placenta
- During the gestation period the fetus develops and grows by gaining the glucose, amino acids, fats, water and oxygen it needs from the mother’s blood
- The bloods run opposite each other, never mixing, in the placenta
- The fetus’s blood connects to and from the placenta by the umbilical cord
- The mother’s blood also absorbs the waste from the fetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the fetus’s blood so that they do not build up to dangerous levels
- Movement of all molecules across the placenta occurs by diffusion due to difference in concentration gradients
- The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion
- The placenta acts as a barrier to prevent toxins and pathogens getting into the fetus’s blood
- Not all toxin molecules or pathogenic organisms (such as viruses, eg rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule)
- This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta
- After the baby has been born, the umbilical cord is cut – this does not hurt as there are no nerves in it, just two blood vessels
- It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind
- The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus – known as the afterbirth
The placenta
Exam Tip
It is worth learning at least two specific substances that move in either direction across the placenta – this is a common exam question and non-specific answers such as ‘waste products’ and ‘nutrients’ will not get any marks!
Stages of Birth
- Amniotic sac breaks
- Muscles in the uterus wall contract
- Cervix dilates (gets wider)
- Baby passes out through the vagina
- Umbilical cord is tied and cut
- Afterbirth is delivered
Antenatal Care
- Antenatal (before birth), care is the name given to the care and advice given to expectant mothers along with checks on fetal growth and development
- Whilst pregnant, expectant mothers are given advice on:
- diet including the need to take folic acid to prevent developmental issues with the fetus and the importance of a balanced diet
- exercise to stay fit
- health precautions such as avoiding infections, tobacco, alcohol and other drugs
Breastfeeding
- During pregnancy the mammary glands enlarge and become prepared to secrete milk
- Shortly after birth, the mother will be stimulated to release milk due to the sucking action of the baby at the breast
- Some mothers struggle to breastfeed successfully and so may feed the baby using formula milk in a bottle
Comparing milk:
