If you have successfully conceived, you should be missing something right about now—your period. A missed menstrual cycle is often the first side effect of pregnancy that women notice, especially if they are not explicitly trying to conceive.
The fourth week of pregnancy marks the start of the embryonic stage of development. After travelling through the fallopian tubes, the blastocyst (a small group of cells) buries itself into your uterine lining, where it continues to grow and develop. By this point, the embryo is about the size of a poppy seed. The blastocyst consists of two types of cells:
- Inner cell mass, which will become fetal tissue and may also be called an “embryoblast”
- Trophoblasts are responsible for nutrient delivery to the embryo and which will eventually form the placenta. The word “trophoblast” comes from the Greek words for “to feed” and “germinator”, trephein and blastos, respectively. This layer of cells is referred to collectively as “the trophoblast”.
The trophoblast begins producing hCG, which signals your ovaries to stop releasing eggs and increase the production of progesterone and estrogen. Placental growth begins and the uterine lining will cease to shed, which is what causes your period to stop. Fetal development begins at this point and your baby’s vital organs are already beginning to form.
The embryo has three layers:
- The endoderm: the innermost layer that will eventually develop into your baby’s pancreas, thyroid gland, gastrointestinal tract lining, and lungs.
- The mesoderm: the middle layer that will grow into sex organs, skeleton, muscles, connective tissues, heart, and circulatory system.
- The ectoderm: the outer layer, which develops into the nervous system, skin, hair, nails, eyes, and teeth.
Buds for the baby’s eventual arms and legs begin to develop, though they will remain indistinguishable for the time being.
Week 4 Ultrasound
Week four is typically the first week when you will be able to see anything in an ultrasound—and it won’t be much. At your fourth week ultrasound, you will be able to see the sac, which will appear as a small, oblong-shaped dark spot.
Changes in You
Symptoms become more pronounced in week four; you may experience one or more of the following:
- Needing to use the bathroom more frequently
- Development of intense cravings and intense dislikes
- Heightened sense of smell
- Mild, occasional cramps
- A metallic taste in your mouth
- Lower abdomen feeling “heavy”
What You Should Be Doing
If you have not done so already, by the end of the fourth week, you should take your first home pregnancy test. If it returns with a positive result, now is the time to make a doctor’s appointment so that they can confirm it. If you do not have a provider, you need to find and choose one. Qualities to consider include the type of professional (obstetrician, midwife, or family doctor) you want to work with and decide if you have any preferences on their gender. You want to be comfortable with this person, as they are going to be an important part of your life for the next 36 weeks, so give it some thought and, if possible, get recommendations from friends and family. Be aware that most medical professionals will not see you before you are eight weeks into your pregnancy unless you have a medical condition or are having irregular symptoms.
The fourth week is the time to stock up on your vitamins and make sure that any medications you are taking are safe for the fetus; this applies to both prescription and over-the-counter drugs.
The End of Week 4
By the end of your fourth week of pregnancy, the placenta will start to function as a means to deliver oxygen and nutrients to the fetus, via the umbilical cord, as well as take away any waste that it produces. This time marks the beginning of the most important stages of fetal development; by now, you should be taking precautions and making any necessary lifestyle changes.