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first trimester of pregnancy

Alternative Names
early antenatal period, pregnancy, first trimester

Pregnancy is the process of gestation. Measured from the start of a woman's last normal menstrual period, or LNMP, it usually lasts about 40 weeks, or roughly 9 calendar months.

The first trimester of pregnancy lies between weeks 0 to 13. Although women experience many of the same physical changes during this time, no two pregnancies are alike.

What is the information for this topic?
A woman can learn if she is pregnant within a few weeks after an egg is fertilised during conception. Pregnancy tests check for rising levels of a specific hormone. Home pregnancy tests may be done on urine. Tests done at a laboratory may use urine or blood.

The fertilised egg is called an embryo for the first 8 weeks. After that it is called a foetus. Its links to the mother appear quite early.
  • The placenta, also known as the afterbirth, begins to form the moment the fertilised egg attaches to the lining of the uterus. It grows inside the uterus, or womb, between the uterine wall and the developing embryo. The placenta is the channel through which oxygen, nutrients, drugs, hormones, and other substances pass from mother to foetus. Waste products from the foetus cross back through the placenta to the mother and are disposed of by the mother's kidneys.
  • The umbilical cord links the foetus to the placenta. The spot at which the umbilical cord connects to the foetus will become its navel, or belly button.
During the first trimester, growth and development in mother and foetus cause many changes to occur. A woman may notice:
  • no period or a light period
  • blue lines under the skin over her breasts and abdomen
  • waistline expansion
  • breasts that grow larger
  • protruding nipples
Other common signs of early pregnancy in women include:
  • nausea sometimes coupled with vomiting known as morning sickness
  • food aversions and cravings
  • heartburn and indigestion
  • fatigue
  • tender breasts
  • complexion problems
  • a need to urinate often
  • constipation
  • headaches, dizziness, or faintness
In the foetus:
  • the heart begins to beat
  • bones appear and the head, arms, fingers, legs, and toes form
  • major organs and the nervous system form
  • the placenta forms
  • hair starts to grow
  • 20 buds for future teeth appear
By the end of the first trimester, the foetus is about 4 inches long and weighs just a bit more than 1 ounce.

While many pregnancies run into no trouble during this time, problems can occur:
  • 10 in 1,000 pregnancies occur outside the womb. Usually a fertilised egg finds a home in one of the fallopian tubes on its way to the uterus. This is known as an ectopic pregnancy.
  • 1 in 5 pregnancies ends in miscarriage, often in the early weeks
During the first trimester of pregnancy, monitoring may include:
  • First month: full physical examination and testing. These tests include a Pap smear, blood tests, urinalysis, and others.
  • Second month: weight and blood pressure and urine test for sugar and protein, ultrasound
  • Third month: height, weight, foetal heartbeat, size, shape and height of the uterus
Author: Dr. Karen Wolfe, MBBS, MA
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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