Your pregnancy week-by-week: Week Four

Welcome to Week 4! This week you may be able to get a positive result on a pregnancy test.

Your Baby

Now that the embryo is nicely implanted into the endometrium of the uterus, it is busy engaged in some serious cell division. Once implanted, the cells split into two, with one part becoming the embryo and the other getting ready to become the placenta. The embryo is barely visible: less than one millimetre long and the size of a poppy seed, and by the end of the week will consist of about 150 cells which are forming layers that will perform specific functions:

  • Endoderm: the inner layer. Becomes the digestive system, liver and lungs.
  • Mesoderm: the middle layer. Develops into the heart, sex organs, bones, kidneys and muscles.
  • Ectoderm: the outer layer. Forms the organs and tissues including the nervous system, hair, skin and eyes.

The Amnion also forms this week: a structure which begins filling with amniotic fluid to protect the growing embryo throughout pregnancy. The Yolk Sac also begins developing: this produces blood and provides nourishment for the embryo until the placenta is ready to take over at around week six of pregnancy. Late, the yolk sac becomes part of the digestive tract.


This week would normally be the last in your menstrual cycle, so you may be expecting your period, though if you are actively trying for a baby, will probably be hoping it doesn’t arrive. For most of this week you will probably not yet know you are pregnant, though you may get a positive result with an early response home pregnancy test. If you test early and get a negative result, do not count yourself out yet – false negatives are common at this stage. You may experience some cramping and a little implantation bleeding, which can be light pink, light red, or light brown and is not a sign that something is wrong.

The hormone hCG is the key to finding out you are pregnant, as it is hCG levels that are measured in a pregnancy test. It is also responsible for the less fun aspects of early pregnancy which may appear this week as your body gets ready for some major changes and to sustain another life. Such side effects include:

  • Feeling easily tired: Take it easy when you feel tired. Your body is adapting to the demands of pregnancy, so let it take some time to adjust – remember, you’re making a new person!
  • Your breasts may tingle and be swollen and tender when hormones stimulate your milk glands. Your nipples may appear larger and darker in colour. Wear a comfortable support bra with no underwire.
  • Morning sickness: You may feel hungry and nauseated at the same time, or may not be able to stand the thought of food. Try eating a few crackers before you rise from bed, eating small bland meals and avoiding high fat foods.
  • Smell or taste aversions
  • Increased need to urinate: You may need to visit the toilet more often as the growing uterus presses on your bladder.
  • Headaches
  • Mood changes: as well as any concerns and fears you have, the pregnancy hormones you are currently mass-producing can turn you into a bit of an emotional wreck. It will even out…eventually.
  • Metallic taste in the mouth.

though you may have noticed many of these symptoms are comparable to how you feel before your period is due anyway, so it can be hard to tell what is going on with your body.

Your Partner

The best thing you can do right now is to be supportive and excited. Be prepared for disappointment if it doesn’t work out: each month brings only a 20-25% chance of conception so it may take a few months. If you’re nervous, discuss it with your partner but try to remain optimistic – parenting is a team effort, even from the outset.

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