Even though the baby is actually only two weeks old, the pregnancy is considered to be a month or 4 weeks along by now.
By the fourth week, days 22-28, the blastocyst turns into an embryo. Once the baby implants into the uterine wall, remarkable things start happening, changes that will continue through the birth. During this week, the baby will continue to divide and grow tiny little body parts will start to take shape, although you still cannot see it very well with the naked eye. The baby is developing their organs and bones layer by layer and all that will make them unique.
Your body is going through changes too, as it prepares for a growing baby. Your belly may bloat and swell, as your breasts may swell and feel tender too. You may feel cramps, which may make you think your cycle is on the way, but actually, it is just the uterus reacting to the new occupant. By the end of this week, your cycle is due, if pregnancy did not occur. Your first real definite sign of pregnancy will show, the missed menstrual cycle.
You can safely take a home pregnancy test by the end of week four, because by now the body is producing enough hCG (human chorionic gonadotropin). This is the hormone first produced by the corpus luteum (the cyst on the ovary that was created when the egg burst forth), then by the placenta, once the baby implants into the uterine wall and the placenta takes over production. Today’s pregnancy tests are so sensitive they can confirm the smallest amounts of hCG in urine. However, if you test and it is negative, that does not mean you are not pregnant. It could just mean your body is not producing enough hCG for the test to detect. Wait two days and test again. hCG doubles every 48 hours.