Development of the Baby at Pregnancy Week 4
After implantation, the blastocyst divides into different cell layers that form the placenta and the embryo (later to turn into the fetus). The embryo consists of two layers called the epiblast and hypoblast . The epiblast layer divides into three essential cell layers – the ectoderm (it develops into the baby’s nervous system, skin and hair), the endoderm (it develops into the gastrointestinal tract, pancreas, liver and thyroid) and the mesoderm (it develops into the connective tissue, skeleton, blood system, muscles and urogenital system) . The hypoblast contributes to the formation of the yolk sac and extraembryonic mesoderm .
The placental cells attach with the uterus lining, establishing blood circulation to the growing embryo so that it gets adequate oxygen and nutrients .
What are the Changes Occurring in Your Body during Pregnancy Week 4?
Your blood volume will start to increase during the 4th week so that your baby receives enough blood supply . Your kidneys need to work harder than usual to process the extra blood which may lead to the symptom of frequent urination . Your heart also has to work harder to pump the additional blood, which may cause tiredness and fatigue. Implantation of the blastocyst leads to production of estrogen, progesterone and hCG hormones  that are responsible for most of the changes in your body.
Tests and Exams for Detecting Week 4 Pregnancy
Implantation triggers hCG (human chorionic gonadotropin) production with its levels being high enough during week 4 for detecting pregnancy with a home pregnancy test . But, it is possible to get negative results in some rare cases where the body produces low amounts of hCG.
The embryo remains too small (the size of a poppy seed)  to be seen on an ultrasound during the fourth week . Although, sometimes an ultrasound scan shows the developing amniotic sac, it is usually too early to detect a heartbeat.
Signs and Symptoms at Pregnancy Week 4
Some women may have no symptoms even during the fourth week. But usually, most of the common early pregnancy symptoms such as nausea, vomiting (morning sickness), dizziness , gas, bloating (due to high progesterone levels) , heartburn, headache, breast tenderness , mood swings, constipation/diarrhea and cramping first occur during this stage. Other symptoms may include:
- Missed period 
- Poor appetite
- Lower abdominal pain
- Lower back pain
- Light vaginal spotting (due to implantation) 
- Shortness of breath
- Food cravings or aversions
- Sensitivity to smells 
- Hot flashes
Although, it is still too early to start showing, some women have larger bellies due to severe bloating.
When to Call a Doctor?
Although it is normal to have the above symptoms during pregnancy, there are certain symptoms that may indicate serious complications such as an ectopic pregnancy or even a miscarriage .
- Severe lower abdominal pain (either on the left or right side) 
- Severe vaginal bleeding
- Visual disturbances
Tips for a Healthy Pregnancy
- Getting an appointment with your ob-gyn as soon as you find out you are expecting
- Consulting your ob-gyn about the risks of any complication or congenital problems
- Following a diet containing plenty of fruits and vegetables rich in folic acid (e.g. oranges, kiwi fruits, spinach)
- Avoiding smoking and drinking alcohol
- Consulting your doctor regarding any medicine (prescription/over-the-counter) you may be taking 
- Asking your doctor for a prenatal vitamin containing iron and folic acid. Most women need higher amounts (600 mcg) of folic acid starting from the fourth week .
- Women with a BMI (body mass index) over 30 may be prescribed a higher folic acid dosage 
- Following a gentle yet regular exercise routine to keep the organs and muscles strong and toned 
- Regular exercise also helps to manage the pregnancy related weight gain
- http://www.huggies.com.au/pregnancy/week-by-week/4-weeks-pregnant [/ref]