The First Trimester

First trimester

Weeks 1 to 13

First Trimester

Weeks 1 – 4: we usually count the 40 weeks of pregnancy duration from the start of your last menstrual period. (If your menstrual cycle is 28 days). Conception occurred on day 14 approximately. This is at the end of the second week.

The sperm and egg combined on Day 14 (range 13-15) inside the Fallopian tube and formed a zygote.

The zygote normally has 46 chromosomes with one copy of 23 being from each biological parent.

The zygote travels to the uterus where it forms a collection of cells, a “blastocyst” and implants and grows into a new fetus. HCG hormone is released, which may cause mild or more severe pregnancy symptoms.

Weeks 6 – 9

In week 6 the fetal heart beat can be detected. The neural tube is developing.

Week 7:  the head develops and the brain and face are in early stages of forming. Retinas form. Limb buds appear.

Week 8: the nose and ears are forming

Week 9 the fetal limbs are growing. The head is still large. The fetus measures around 16 to 18 mm and this will double in the next two weeks. All the major structures are now growing.

At 13 weeks your baby is about 7 cm long, and is the size of a lemon.

The brain is forming and the baby can move all the limbs.

Symptoms experienced may include:

  • Extreme tiredness
  • Tender and swollen breasts and nipples
  • Nausea with or without vomiting
  • Mood swings
  • Constipation
  • Frequent bladder emptying
  • Cravings or distaste for foods and smells


You need to have the pregnancy confirmed by your GP and some early pregnancy tests arranged.

Women are recommended to take folate and iodine supplements from as early as possible for healthy fetal development.

Your doctor may recommend additional supplements, especially if your diet is limited.

If you are vomiting and dehydrated you should seek medical attention.

A small amount of bleeding can occur, often without any adverse problem, sometimes this is referred to as implantation bleeding. Contact Dr O’Shea’s team during work hours if this has occurred.

For heavy bleeding, it is best to attend the Emergency Department or after hours GP.

Medical treatment of nausea may be advised especially if vomiting is severe or prolonged.

Dr O’Shea will discuss with you screening for chromosomal abnormalities. and arrange testing if appropriate.

Available tests include an NIPT (blood test) done at weeks 11- 14, or a Nuchal Translucency (NT) scan, done at weeks 11.5 to 13.5.

Below are some useful links.