Health encyclopaedia

Health encyclopaedia - Alphabetical Topic List

Pregnancy

Pregnancy occurs when a sperm penetrates an egg.  The single sperm that first burrows into the egg is the successful one of millions of others 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). 

Sperms may contain an X chromosome or a Y chromosome. If an X fertilises, the result will be a girl; if a Y penetrates, the result will be a boy.  Sperms and eggs each contain 23 chromosomes, and the total number required of 46 chromosomes is achieved when a sperm and egg fuse at fertilisation. 

The fertilised egg immediately begins to divide into a growing cluster of cells.  Between five and seven days after ovulation, the fertilised egg implants into the wall of the uterus and produces root-like outgrowths called villi.  These help to anchor it to the lining of the uterus 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 growing baby.

From the time of implantation into the wall of the uterus until approximately the eighth week of life, the 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 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 uterus.  A fluid-filled double membrane surrounds the baby, and during pregnancy it floats freely in the amniotic fluid.  The baby constantly swallows this fluid and excretes it in the urine, so much information about the health of the baby can be obtained during a test called Amniocentesis.  The membrane normally breaks and releases the amniotic fluid before the baby is born.

The length of a normal pregnancy varies between about 37 and 42 weeks, although the delivery (term) date is calculated at 40 weeks from the first day of the last period.  Only around five per cent of babies are born on their due date.  Babies born before 37 weeks are considered premature (or pre-term).

• Symptoms
• Diagnosis
• Definition
• Complications

• Paget's diseasePainkillersPancreatitisPanic disorderParkinson's diseasePatient Advice and Liaison ServicesPatient and Public Involvement ForumsPatient choicePelvic inflammatory diseasePemphigus vulgarisPeptic ulcerPerforated eardrumPericarditisPeriodsPeriods, absentPeriods, heavyPeriods, irregularPeriods, painfulPeritonitisPersistent vegetative statePET scanPhenylketonuriaPhobiasPhosphate testPhysiotherapyPilonidal sinusPlasma productsPlastic surgeryPleurisyPneumococcal infectionsPneumoniaPoisoningPoliomyelitisPolycystic ovarian syndromePolymyalgia rheumaticaPolyps, nosePost-mortemPost-natal depressionPost-traumatic stress disorderPotassium testPre-eclampsiaPreconceptionPregnancyPremenstrual syndromePreoperative testsPressure ulcersPriapismPrickly heatPrimary biliary cirrhosisProgestogen-only pillProlapse of the uterusProstate diseasePsoriasisPsychiatryPsychosisPsychotherapyPuberty

  ---