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.
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 .
Your baby is about the size of an orange seed this week (around 2 mm long)  while his shape resembles that of a tadpole 
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.
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 .
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 .
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:
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 .
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 .
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:
Published on November 29th 2013 by Pregmed Editorial Team.
Article was last reviewed on 13th August 2014.