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You might have noticed that pregnancy is talked about as being 40 weeks in duration. In fact, full-term pregnancy is considered to be between 37 and 42 weeks. However, most of the information on pregnancy itself seems to start around week 4, so what is happening in those initial 4 weeks?
Let's be clear, to identify the exact date at which conception occurs is nearly impossible. Even if you know that you only had sexual intercourse on one particular day that month, that date is not necessarily your date of conception. This is because it can take the sperm up to a day to reach the egg, or the egg may leave the sperm waiting for several days. For these reasons, your pregnancy duration will be measured from the first day of your last menstrual period. So, during week 1 you are not actually pregnant, but having your period; and neither are you pregnant in week 2. The week following your period your body gets busy thickening the lining of your uterus all over again; this is where the fertilised egg will eventually be embedded. During week 2 your ovaries are also hard at work preparing for ovulation (the release of one or more eggs into one of your fallopian tubes). Conception will occur when a sperm meets the egg (or eggs if you are going to have fraternal twins, triplets or more), and this usually occurs in the fallopian tube. The fertilised cell divides repeatedly and becomes a ball of cells known as the blastocyst. On the basis of a 28-day cycle, it is generally assumed that conception occurs on day 14. During week 3 the blastocyst travels down to the uterus and during week 4 it will become embedded in the uterus lining (the endometrium).
Once it has become embedded, the ball of cells will divide further into two groups, one which will go on to grow into the foetus (your baby) and another which will grow to become the placenta which provides the nutrients and an effective waste facility for your baby for the rest of your pregnancy. From the time of implantation (where the cells embed in the lining of the uterus), your body begins to produce a hormone called human chorionic gonadotropin (hCG). This is what is detected in a pregnancy test. A blood pregnancy test can detect this hormone earlier than a urine test but blood tests are not used routinely as they are far less convenient. Home pregnancy tests that detect hCG in your urine are simple and affordable, and in general they will work from the first day of your missed period (the end of week 4), but some are very sensitive and can show a positive result sooner than this.
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