Let us look at this pregnancy

The effects of pregnancy hormones will bring early symptoms. Fullness and tenderness in the breasts are the most common complaints this week. Sleepiness and weakness are also common. The growing uterus reduces bladder capacity, so you may need the toilet frequently. If you have not yet seen your doctor, arrange a visit: it is time to meet the developing life inside you. Vaginal ultrasound can show the gestational sac in the uterus. If no sac is visible, an ectopic pregnancy is possible. This is a serious, potentially life-threatening condition, so do not delay your first appointment.

Development accelerates on day 16 after fertilisation. The endoderm forms the membranes around the lungs, tongue, tonsils, urethra, bladder and digestive system. The mesoderm gives rise to muscles, bones, the lymphatic system, spleen, blood cells, heart, lungs, and reproductive and excretory systems. The ectoderm gives rise to the skin, nails, hair, eye lens, hearing system, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, breast tissue and nervous system. At 17-19 days after fertilisation, the embryo resembles a pear, with a broader head end than tail end. The ectoderm thickens to form the neural plate. Its edges fold to form the neural groove, the beginnings of the nervous system and one of the first organ systems to develop.

By the end of this week, blood cells have formed and channels are developing between epithelial cells. On day 21, mesoderm-derived somites appear along both sides of the neural groove, beginning at the tail end. Endocardial cells have formed the primitive heart tubes.