3 weeks pregnant: Key points

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Changes for mum

  • Fertilisation, should it occur, usually does so on the first day of week 3 of pregnancy, 15 days after your last menstrual bleeding began and 24 hours after ovulation, which typically occurs on the last day of week 2.
  • After ovulation, the egg must come into contact with one of your partner’s sperm for fertilisation to occur. It sends out chemicals to attract sperm.
  • When a sperm comes into contact with your egg, it releases chemicals which help it penetrate the egg and break through the shell. The egg and sperm both release chemicals to ensure only one sperm penetrates the egg.
  • The sperm and egg combine their chromosomes (23 each) to form a single, 46 chromosome cell which contains all the genetic information needed to develop into your baby. The single cell will produce over two trillion new baby cells before you give birth.

Changes for baby

  • The fertilised egg, known as a zygote travels through your fallopian tube towards your womb.
  • 4-5 days after fertilisation, your baby has grown to a size of 13-32 cells and is known as a morula. It’s less than a millimetre in length and looks something like a tiny raspberry.
  • The morula enters your womb and becomes known as a blastocyst. It releases chemicals which eat away at the lining of your womb to make a space where it can implant and obtain the nutrients it needs to grow into a healthy baby.
  • The blastocyst is attached to the womb only superficially and there is a high risk of miscarriage at this stage. However, a very early miscarriage appears the same as normal menstrual bleeding- if it does occur you won’t know.

Pregnancy health

  • Sexually transmitted infections and other infections can interfere with the processes by which your fertilised egg implants in your womb. Make sure you are tested for and treat STIs before trying to get pregnant.
  • Avoid harmful substances like alcohol and tobacco throughout your pregnancy but especially this week. Your pregnancy is still establishing itself and vulnerable to miscarriage. Dangerous substances may increase the risk of an early miscarriage.
  • Keep exercising- unless you’re training for a marathon, exercise won’t disturb the implantation of your fertilised egg.

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More information on the 3rd week of pregnancy

For more information on the third week of pregnancy, see 3 weeks pregnant.
For more information about the changes that occur when pregnant, see Changes to mum this week.
For more information about what happens to create your growing baby after conception, see Changes for baby this week.
For more information about preparing your body for pregnancy with healthy eating and lifestyle changes, see Pregnancy health information this week.
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Date Created: August 22, 2012 Date Modified: March 15, 2013