Week 7

In the first few weeks of the baby’s life, brain growth is astonishing! At week 7, the brain starts producing neurons at the incredible pace of 250,000 neurons per minute. ^{18 19} This is especially shocking given that the brain essentially stops creating new neurons about 2 months before birth.^{20 21}

By the end of week 7, the brain has separate right and left cerebral hemispheres.²² The cerebral hemispheres will direct speech, decision making, movement, balance, vision, memory, and many other functions. Seven weeks gestation is also the first time that substance P and enkephalins, two signaling molecules in the nervous system, also known as neurotransmitters, dedicated to processing pain, have been detected.²³

The shift to placental nutrition happens gradually. Before 7 weeks of gestation, the baby relies mainly on the yolk sac and uterine lining for nourishment while the placenta continues to form.²⁸ By week 7, the placenta and umbilical cord are developed enough to take over the primary role of delivering oxygen and nutrients to the unborn baby and removing wastes like carbon dioxide.²⁹ By 10 weeks, the placenta is the primary source of life, providing everything the unborn baby needs to survive.³⁰ Before the placenta is mature, the baby has less oxygen. Fetal blood can carry roughly 40% more oxygen than maternal blood, allowing the baby to thrive in low-oxygen environments.³¹ Overall, the placenta’s life-support capabilities create an irreplaceable bond between the mother and child.

In gestational week 7, the oropharyngeal membrane, a membrane that closed off the digestive tract, disintegrates. Now the amniotic fluid can enter the unborn baby’s mouth and digestive system. Furthermore, muscle cells start growing to control facial expressions and jaw movements.³² External ears also appear, and the eyes begin to move toward the front of the face as the baby’s head grows and develops.³³

Between 300 and 1300 primordial germ cells migrate from the yolk sac to the developing gonad which will become the ovaries or testes.³⁸ These germ cells will differentiate into early sperm cells called spermatogonia in males and early egg cells called oogonia in females.³⁹ In a woman’s body, oogonia give rise to primary oocytes or “eggs”, which remain dormant in the ovaries for up to 40 years. Interestingly, no primary oocytes form after birth; a woman’s body creates all of her eggs starting at seven weeks gestation and finishing around five months.⁴⁰ Similarly, in a man’s body, the primary germ cells that migrate in week 7 become spermatagonia, which will create all the man’s sperm.⁴¹ Interestingly, a single oogonia becomes one egg while a single spermatagonium becomes 4 sperm.⁴²

Bladder development begins around week 7.  The bladder first forms as a ‘cloaca’, a common chamber for the early kidneys and gut. By 10 weeks, the cloaca has divided into two tubes; one becoming the end of the intestines, and one becoming the urethra to help empty the bladder.^43 44 As early as 9 weeks, ultrasound has confirmed the presence of urine within the bladder, showing functional kidney activity.⁴⁵ By 14 weeks, subtle anatomical differences emerge between male and female babies, with changes in the position of the bladder.⁴⁶ Interestingly, in both infants and young children, the bladder remains located within the abdominal cavity. It does not descend into the pelvis until around six years old, ultimately reaching its adult position during puberty.⁴⁷