Week 8 to 9

Complex behaviors

Human Prenatal Age
  • PCW 9
  • Days 56-62
  • Gestational Week 11
  • The unborn child is officially called a fetus.

  • The fetus starts producing substance P, a neurotransmitter specific to pain signaling.1

  • External genitalia start developing into a penis or a clitoris and labia majora. A female develops a distinct uterus. Her future egg cells are replicating in the ovaries.2

  • Red blood cells start forming in the liver.3

  • Tooth buds are developing.

Once the preborn baby starts moving, he doesn’t keep still! When researchers quantified fetal movements, they found that the fetus does not stay still for more than 13 minutes at a time.4 The fetus starts showing more complex behaviors, such as thumb-sucking, swallowing, and stretching.5 These complex behaviors reflect the explosive growth in the developing brain, growing at over 250,000 neurons per minute.6

By nine weeks after conception, the nerve receptors inside the fetal skin can sense light touch. If something lightly tickles the sole of the fetus’s foot, the fetus will bend his knee to withdraw his foot, and may curl his toes.7

This image shows a real living embryo, recorded using fetoscopy. The picture was safely taken while he was still in the womb. At this point, he rarely stays still. (Image Credit: The Center for Bio-Ethical Reform)
What is the difference between an embryo and a fetus?

Prenatal development is continuous. There is no clear feature distinguishing an embryo from a fetus. Historically, if bone marrow had started replacing cartilage within the leg bone called the humerus, then the embryo was considered a fetus.8 Now, nine weeks after conception, the embryo is called a fetus. A fetus has developed every major body organ, though they may not be in their final location, and they may not be fully functional yet.9

What is the amniotic sac?

The amniotic sac is a thin but tough pair of transparent membranes, which hold the developing baby until shortly before birth. The inner membrane, called the amnion, contains the amniotic fluid and the fetus. The amnion does not have any blood vessels of its own. The other membrane, called the chorion, keeps the amnion safe and is part of the placenta. The chorion contains fetal blood vessels.10

Before the fetal skin thickens, water and nutrients from the amniotic fluid actually pass through the skin to the fetus. Fluids from the fetal digestive tract and respiratory system enter the amniotic fluid as well. Starting around 11 weeks after conception, the fetus starts urinating into the amniotic fluid, too. Given that the amniotic fluid penetrates the developing fetus at many locations, it must be kept clean. In fact, the water content of the amniotic fluid changes every three hours! Large amounts of water pass through the amniochorionic membrane at the placenta to keep amniotic fluid in balance with fetal circulation.11

The volume of amniotic fluid increases slowly, reaching around 30 milliliters around 10 weeks after conception, 350 milliliters around 20 weeks after conception at up to 1 liter by the time the baby is born.12

This baby and his placenta are surrounded by the amniotic sac, a fluid-filled sac designed to protect and nourish the growing child. (Image Credit: Priests for Life)
Notice how the toes have all separated and can wiggle freely. (Image Credit: Priests for Life)
When is the baby’s sex determined?

The baby’s sex is determined at conception by the combination of X and Y sex chromosomes that the baby receives from the egg and the sperm. All eggs contribute an X chromosome, while sperm contribute either an X or a Y chromosome. Embryos with XY chromosomes develop male sex organs, while those with XX chromosomes develop female sex organs. Therefore, the father’s sperm determines a baby’s sex.13

How do male and female babies develop differently?

For the first few weeks of development, all embryos have both male and female internal reproductive structures, and their external genitalia remains similar for the first eight weeks after conception. Between 6 and 10 weeks after conception, the baby’s internal sex organs start to develop. The Y chromosome signals for male hormone production around 5 weeks after conception. These hormones act on the reproductive structures, turning them male. Although rare, there are various genetic and developmental disorders that can occur during sex development.  By 10 weeks after conception, the external genitalia start to lengthen in males to form the penis, or remain short in females to form the clitoris and labia. Doctors can usually tell if the baby is a boy or a girl by using an ultrasound by 12 weeks after conception.14

Dive Deeper
Around 9 weeks after sperm-egg fusion, the external structures start to differentiate.
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion