Week 4

The circulatory system also starts developing remarkably early.  Early blood vessels start to form a network throughout the unborn baby by 18 days after conception.⁸ Remarkably, the embryo’s earliest heart cells start to develop even earlier, by 16 days after conception.⁹ Heart cells start forming a simple tube.¹⁰ By the end of the week, some of the early heart muscle cells begin to twitch, but their movements are not yet coordinated.¹¹ By 22 days after conception, the heart starts beating rhythmically.^{12 13}

Interestingly, blood forming cells start growing outside the embryo in the umbilical vesicle, also known as the yolk sac, around 18 days after conception. These blood forming cells move into the liver about 5 days later, then finally settle in the bone marrow at 10 ½ weeks after conception.¹⁴ Further clusters of blood-producing cells can be found near the embryonic aorta between days 27 and 40 after conception.¹⁵ The liver becomes the primary blood-forming organ from about 8 weeks until 24 weeks gestation.¹⁶

At 18 days after conception, scientists observe the first sign of the developing brain. Neuroectoderm tissue thickens and forms the neural plate. The neural plate will fold together to become the neural tube, which later becomes the brain and spinal cord.¹⁷

The placenta and umbilical cord are the lifeline for the rapidly developing baby. Oxygen and vital nutrients flow from the mother to the unborn child, while harmful wastes like carbon dioxide are carried back to the mother through placental circulation. Just 13 days after conception, blood flow between the uterus and placenta has begun.²³ The embryo, as pictured above, is connected to the early placenta via a connecting stalk. The connecting stalk later develops into the umbilical cord.²⁴ The placenta forms through an intricate interaction between maternal and embryonic tissues. Perhaps most fascinating of all, the placenta is the only fetal organ shared by mom and baby. The placenta carries the baby’s DNA but is intimately embedded in the mother.²⁵