Pregnancy occurs when a sperm penetrates an egg. The single sperm that burrows into the egg is the successful one out of millions contained in the man's semen. This is called fertilisation, and usually takes place in the woman's fallopian tube after the matured egg has been released from one of her ovaries (ovulation).
Sperm may contain an X chromosome or a Y chromosome. If an X fertilises the egg, the result will be a girl. If a Y penetrates the egg, the result will be a boy. Sperm and eggs each contain 23 chromosomes. The total number of chromosomes required to make a baby is 46, and this is achieved when a sperm and egg fuse at fertilisation.
The fertilised egg, known as an ovum, immediately begins to divide into a growing cluster of cells. Between five to seven days after ovulation, the fertilised egg implants itself into the wall of the womb (uterus) and produces root-like outgrowths called villi. These help to anchor it to the lining of the womb and will eventually grow into the placenta, the organ that feeds and protects the baby until birth.
The placenta maintains and nourishes the baby by enabling the transfer of oxygen, carbon dioxide, amino acids, fats, vitamins and minerals from the mother's blood. It also allows the transfer of waste substances from the baby.
Embryonic and foetal stages
From the time of implantation into the wall of the womb until approximately the eighth week of life, the developing baby is known as an embryo. Development is rapid during this stage, as specialised cells begin to form the vital organs, nervous system, bones, muscles and blood.
After the eighth week of pregnancy, the developing baby is called a foetus. It is about 2.4cm (1 inch) long, with most of the internal organs formed. External features, such as the eyes, nose, mouth and ears, can be seen, and fingers and toes start to appear.
As the unborn baby grows, so does the womb. A fluid-filled double membrane surrounds the baby. This normally breaks when the baby is ready to be born, releasing the amniotic fluid (the liquid that surrounds the baby).
During pregnancy the baby floats freely in the amniotic fluid, and constantly swallows this fluid, excreting it in their urine. Much information about the baby's health can be obtained during a procedure called amniocentesis, where a small sample of amniotic fluid is taken for testing.
The length of a normal pregnancy varies between about 37 and 42 weeks, although the delivery date is calculated at 40 weeks from the first day of your last period. Only around 5% of babies are born on their due date. Babies born before 37 weeks are considered premature (or pre-term).
Defining pregnancy
The first trimester
Weeks 0 to 13 are known as the first trimester of pregnancy. The baby's development is greatest during this stage and by the end of week 13, it will be fully-formed. Some of the common discomforts of pregnancy, such as nausea, fatigue, and breast tenderness, will be most pronounced during these early weeks.
Symptoms of pregnancy
Some signs and symptoms to look for if you believe you might be pregnant include
missed period,
nausea with or without vomiting,
tiredness,
dizziness,
breast changes and breast tenderness, and
frequent urination.
Feelings of sickness (nausea) affect the majority of women during pregnancy, particularly during the first three to four months. Tiredness may be more pronounced during the first and third trimesters.
Diagnosis
If you suspect that you are pregnant, or have taken a home pregnancy test which confirms that you are pregnant, you should arrange to see your GP or midwife straight away.
This is to ensure that you receive all the necessary advice and support you need, as early as possible. Midwives may be based at your GP's surgery, a local hospital or midwife unit.
The diagnosis of pregnancy is mainly determined by testing for the pregnancy hormone, human chorionic gonadotropin (HCG). HCG can be detected in the woman's blood and urine even before the first missed period.
It is this hormone that is measured by home pregnancy tests. A high HCG level may indicate a multiple pregnancy, such as of twins or triplets, although the most reliable test is an abdominal, or transvaginal scan.
Abdominal or transvaginal scans may be used to confirm pregnancy, how many embryos are present, and to check that the embryo is located in the womb, rather than in the fallopian tube, as in the case of an ectopic pregnancy.
Some signs and symptoms to look for if you believe you might be pregnant include
missed period,
nausea with or without vomiting,
tiredness,
dizziness,
breast changes and breast tenderness, and
frequent urination.
Feelings of sickness (nausea) affect the majority of women during pregnancy, particularly during the first three to four months. Tiredness may be more pronounced during the first and third trimesters.
There are a number of things that you can do during your pregnancy to lower the risk of problems occurring, and help keep you and your baby healthy.
Exercise
Moderate exercise is recommended for most pregnant women. It helps boost energy levels, improves your overall health, and helps prepare you for labour by increasing your muscle strength and stamina.
Your midwife or GP will be able to recommend how much and what sort of exercise is good for you. Generally, 30 minutes of moderate daily exercise is recommended for pregnant women during most of their pregnancy. Activities like walking or swimming are ideal.
You should avoid contact sports, exercises that require you to lay flat on your back, and any extreme sports or activities that may put you at risk of falling over. Scuba diving is not recommended while pregnant because it can damage the baby's development.
Things to avoid
Some other things that you should avoid while pregnant that may put your baby's health at risk are outlined below.
Smoking - smoking increases the chances of your baby being born under-weight or being born too early - both of which could harm their health. Your midwife, or GP, will be able to give you advice about quitting smoking, or you can call the NHS Pregnancy Smoking Helpline on 0800 169 0 169.
Alcohol - the Department of Health recommends that you should not drink alcohol while you are pregnant because it can harm your baby. You should also avoid drinking alcohol if you are trying for a baby. If you do drink when you a pregnant, you should not drink more than one to two units of alcohol, once or twice a week (one unit is roughly a small glass of wine, half-pint of ordinary strength bitter or lager, or a pub measure of spirits).
Cannabis - the effects of cannabis on unborn babies is unknown, so it is impossible to say what smoking cannabis could do to your baby.
Prescription and over-the-counter medicines - only a few prescription and over-the-counter (OTC) medicines have been shown to be safe for pregnant women by good quality studies. Your GP will only prescribe you a prescription medication when it is felt that the benefit outweighs the risk. You should use as few OTC medicines as possible.
Complementary therapies - very few complementary therapies have been proved to be safe and effective for pregnant women. You should not assume they are safe and use them as little as possible.
Shark, swordfish and marlin - you should avoid eating these types of fish as they all contain high levels of mercury, and therefore could affect your baby's developing nervous system.
Diet
While you are pregnant, you should try to follow a healthy, balanced diet based around the four main food groups. For example:
four to six daily portions of carbohydrates, such as rice, pasta, cereals, bread and potatoes,
at least five daily portions of fruit and vegetables,
three daily portions of protein from sources such as meat, poultry, fish, pulses and eggs, and
at least one portion a day of pasteurised dairy products, such as milk, cheese and yoghurt.
Eating fish is very healthy for you, and your unborn baby. However, you should avoid eating shark, marlin and swordfish, as these fish contain high levels of mercury, which can affect your baby's development.
Also, limit the amount of tuna fish you eat to no more than two tuna steaks or four medium-sized cans per week.
You should drink plenty of fluids such as water and fruit juice, preferably at least two litres every day. This will help your body to get rid of toxins and waste products, and it can also help to prevent constipation and nausea.
You should limit the amount of caffeine you have, as too much caffeine can lead to your baby having a low birth weight, which can cause them health problems later on.
Caffeine
The Food Standards Agency recommends no more than 200mg of caffeine per day during pregnancy. This is equivalent to:
two mugs of instant coffee,
one mug of filter coffee,
two mugs of tea,
five cans of cola, or
four (50g) bars plain chocolate (milk chocolate contains less caffeine than plain chocolate).
Some cold and flu remedies contain caffeine, so always check with your pharmacist before taking any.
Vitamins and supplements
Your GP or midwife should give you information about taking folic acid. This helps reduce the risk of your baby having conditions which are known as neural tube defects, such as spina bifida. The recommended dose is 400 micrograms a day from as soon as you think you may be pregnant (or ideally when you begin trying for a baby).
You should not take vitamin A supplements as they can cause abnormalities in unborn babies. Liver is high in vitamin A, so you should avoid that too.
Food hygiene
Your GP or midwife should give you information on how to avoid bacterial infections from food, such as listeriosis and salmonella, which could harm your baby. You should avoid:
mould-ripened soft cheese, such as Camembert or Brie, and blue-veined cheese (there is no risk with hard cheese, such as Cheddar), or with cottage cheese or processed cheese,
pâté, including vegetable pâté,
uncooked or undercooked ready-prepared meals,
raw or partially cooked eggs, or food that may contain them, such as mayonnaise, and
raw or partially cooked meat, particularly poultry.
Toxoplasmosis is an infection that can occasionally cause problems for unborn babies. You can pick it up from undercooked or uncooked meat such as salami, which is cured, and from the faeces of infected cats or contaminated soil or water.
You should ensure that you:
wash your hands thoroughly before handling food,
wash all fruit and vegetables before eating them, including ready-prepared salad,
thoroughly cook raw meat and ready-prepared chilled meals,
wear gloves and wash hands thoroughly after gardening or handling soil, and
avoid contact with cat faeces, either in cat litter or in soil.
Pregnancy
