Week 9

The taste system starts forming in week 8 when nerves enter the unborn baby’s tongue and bumps start forming on it.¹³ These nerves both help tastebuds to form, and send signals from the tastebuds to the brain. ¹⁴ In week 9 early tastebuds start forming.¹⁵

Each tastebud is like a tiny basket made up of 50 to 100 taste receptor cells that work together to detect the flavors in food. Taste receptor cells don’t last long — they only live for about 10 days before being replaced by fresh ones.¹⁶ But during their short lives, they pass along taste signals to the brain through nerve connections at the base of the taste bud that can be observed as early as 10 weeks gestation.¹⁷ By week 11, about a quarter of tastebuds have already connected to early nerve fibers.¹⁸ Each taste bud contains multiple types of taste receptors, so most can detect sweet, sour, salty, bitter, and savory flavors.¹⁹ There are at least two different types of taste bud cells by 14 weeks.²⁰

The baby’s sex is determined at conception by the combination of X and Y sex chromosomes that the baby receives from the egg of the mother and the sperm of the father.  If a baby has an XY chromosome, he is male and this is the week that testes start to form. The Y-chromosome carries a special gene called SRY required for male development and directing the baby’s body to form testes instead of ovaries. Once the testes form, they produce three hormones: testosterone, dihydrotestosterone, and AMH. All three of these hormones are essential for male genitalia formation.²¹ Therefore, the Y chromosome acts like a switch for male development. Without these hormones, the baby develops with female characteristics.²²

In week 9 of pregnancy, toes start to form, and the tissue between the fingers becomes thin.²³ As the arms and legs lengthen, the hands and feet approach each other in front of the unborn baby.²⁴ Over the course of week 9, the limbs rotate so that the hands and feet can face down as well as facing towards each other.²⁵

The unborn baby’s bones begin to form “bone models” consisting of soft, flexible tissue called cartilage—the same tissue that gives the outer ear its structure. By the end of week 9, the entire limb skeleton is made of cartilage.²⁶ Between weeks 8 and 9, cartilage starts to harden and become bone in a process known as ossification. This begins in the center of each future bone and gradually spreads outward.²⁷ Some bones, like the skull and collarbone, form directly from sheets of tissue rather than cartilage.²⁸ The first bones to form in the baby are the collarbone and thigh bone.²⁹ Knees and elbows also start developing.^{30 31}

The baby’s face forms quickly and is recognizable by about week 9. It begins around week 6 as small tissue bulges that grow and fuse to form the forehead, nose, and mouth.³² The eyes also take shape as the outgrowths from the developing brain form the optic nerve and back of the eye. These structures interacted with the outer layer of the embryo and formed the eye’s lens.³³ Between weeks 7 and 8, the nose, nostrils and lips became more defined.³⁴ Now in week 9, the face looks similar to a newborn’s face. However, the area between the upper lip and nose is still forming. If this area does not fully close, it can lead to cleft lip or cleft palate, which can be surgically repaired after birth.³⁵

At 9 weeks of pregnancy, the unborn child’s heart rate peaks around 170 beats per minute, making baby’s heartbeat almost twice as fast as mom’s.³⁸ The baby’s heart has almost completed forming its four chambers and major vessels. This week the chamber walls finish growing to separate the chambers.^{39 40 41} Specialized cells called neural crest cells invade the developing heart and help create the nerves that will direct changes in heart rate. These cells also help create smooth muscles inside the aorta and arteries leading to the lungs, which helps regulate blood pressure.⁴²