This is the week when your baby’s heart and nervous system starts to develop while the hCG level in your body is finally high enough to get a positive home pregnancy test result.
Development of Your Baby at Pregnancy Week 5
The three layers – the ectoderm, endoderm and mesoderm – forming the embryo begins developing into different organs and body parts.
Ectoderm Layer: Ectoderm Layer: Forms the neural tube which later sprouts the baby’s brain, nerves, spinal cord and backbone . The somites (body segments) start to develop that later forms the skeletal muscle, cartilage, tendons and dermis.
Mesoderm Layer: At the same time, the circulatory system and the heart start developing. The first red blood cells (RBCs) of the baby also start developing at this stage. However, the yolk sac continues to take care of the baby’s needs as the heart is not developed enough to take charge .
Endoderm Layer: Grows into the lungs, intestines, rudimentary urinary system, thyroid gland, pancreas and liver . The developing blood vessels build a connection between you and your baby, which later turns into the umbilical cord . The primitive umbilical cord and placenta send all the nourishment and oxygen your baby needs  while the amniotic sac continues to grow .
How Big is Your Baby?
Your baby is about the size of an orange seed this week (around 2 mm long)  while his shape resembles that of a tadpole 
What Happens in Your Body during Pregnancy Week 5?
Some women do not feel any change in their body during the fifth week, even though the baby undergoes significant development. This is the week when most women first notice that they are late for period and opt for a home pregnancy test . The estrogen, progesterone and hCG levels continue to rise during this week , which often leads to various symptoms. Like the previous week, your blood volume will continue to increase this week, causing your kidneys and heart to work extra hard.
What Does 5 Weeks Pregnant Look Like?
Although, the body changes are enough to let you know your baby is growing inside you, it is still too early to start showing a pregnancy belly .
Tests and Ultrasound for Detecting Pregnancy at Week 5
Transvaginal ultrasound is more efficient than transabdominal ultrasound in detecting pregnancy in the first trimester . Ultrasound scan performed during the fifth week may be able to detect the baby’s heartbeat as tiny movements as it is still too low to be heard .
Home pregnancy tests give positive results as the hCG levels normally become high enough to be detected in the urine at this stage . However, it is possible to get a negative pregnancy test in case of low hormone production. The pregnancy hormone levels are usually highest when you wake up in the morning, so, it is the best time to have the test.
Your doctor may also recommend a blood test to determine the hCG levels as it helps to find out about the health of the embryo .
What are the Symptoms of Week 5 Pregnancy?
Most women start experiencing the earliest pregnancy symptoms at week 5. However, there is nothing to worry about if you have no symptoms. Common early pregnancy symptoms include nausea or queasiness  and vomiting (morning sickness), headaches, breast tenderness, bloating, heartburn , constipation/diarrhea, abdominal tightness, gas and indigestion. Additional symptoms that may persist or come and go frequently are:
- Heightened sense of smell
- Feeling full or heavy all the time (due to the additional blood supply to the uterus and the pelvic engorgement) 
- Dizziness and feeling of faintness (may worsen if you have low blood sugar or have not eaten in hours)
- Feeling out of breath (more common in twin pregnancies) 
- Dysgeusia (metallic taste in the mouth) 
- Food cravings or aversions
- Tiredness or fatigue (due to the hormonal changes and the growing placenta) 
- Menstrual-like cramping and lower back pain 
- Night sweats 
- Frequent urination (due to the increased blood flow to the kidneys and pelvic region
- Excessive saliva (caused by the functioning of the pregnancy hormones)
- Brown vaginal discharge or spotting (implantation bleeding) 
- Clear jelly-like vaginal discharge
- Emotional changes or mood swings
- Excessive thirst 
- Leg and foot cramps
- Joint and groin pain (mild)
- Rhinitis of pregnancy causing stuffy/runny nose (due to the high estrogen levels) 
Twin pregnancies produce higher levels of pregnancy hormone compared to singleton ones; so, the symptoms of morning sickness may be more severe in women carrying twins .
What Congenital Defects Can Occur during Pregnancy Week 5?
Most serious birth defects, including those involving the neural tube, occur during the first trimester. Any abnormality in the tail side of the neural tube can cause spina bifida while abnormalities in the head side can affect the skull formation, leading to anencephaly .
When to Call the Doctor?
Make sure to watch out for the following symptoms as they may indicate serious complications such as ectopic pregnancy, a miscarriage or a bacterial infection:
- Severe vaginal bleeding 
- Passing blood clots
- Extreme dehydration (may indicate conditions like gestational diabetes)
- Severe lower abdominal pain or cramping (left or right side)
- Extreme lightheadedness 
- Extreme depression (lasting for over two weeks) 
Tips for a Healthy Pregnancy and Baby
- Following a diet containing lots of vitamin C-rich fruits such as orange, kiwi fruit and blackcurrant as vitamin C is beneficial for your baby’s rapidly growing cells 
- Continuing your prenatal vitamin containing vitamin D and folic acid
- Giving up caffeinated drinks and alcohol 
- Giving up smoking
- Continuing your jogging routine if you used to jog daily before conceiving (it is safe to run 3-4 days a week), unless you have a high risk pregnancy 
- Swimming for a short time every day for maintaining the basic fitness level 
- Informing your health care practitioner about your pregnancy before undergoing an x-ray exam for any ailment (e.g. dental)
- Chewing a flavored sugarless gum to manage the symptom of excessive saliva production 
- Consulting your doctor if you take any drugs potentially harmful for the baby (e.g. antianxiety drugs like xanax and anticoagulants like warfarin)
- http://www.babycentre.co.uk/a7877/jogging-and-running [/ref]