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 .
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.
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.
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:
Although, it is still too early to start showing, some women have larger bellies due to severe bloating.
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 .
Published on November 29th 2013 by Pregmed Editorial Team.
Article was last reviewed on 13th August 2014.