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)
How do male and female babies develop differently?

In the first few weeks of development, all embryos have the same internal and external genitalia. Internally, embryos develop 3 tubes connecting to a central opening. One tube is the ureter, which carries urine from the developing kidneys and bladder. The other two tubes will serve to move either the sperm or egg.

Starting 5 weeks after conception, an embryo with a Y chromosome starts creating testes determining factor. This chemical messenger helps males develop testes. The testes produce two important hormones: testosterone and anti-Mullerian hormone.13

In males, testosterone helps the male tube survive while the female tubes shrivel. The male tube develops into the vas deferens. Conversely, in females the lack of testosterone helps the female tube survive while the male tubes disintegrate. The female tube later becomes the fallopian tubes and cervix.14 In the ninth week after conception, females start developing a small uterus.15

Notice how the toes have all separated and can wiggle freely. (Image Credit: Priests for Life)

Externally, both male and female embryos have similar genitalia for the first seven weeks. Both sexes develop folds of tissue adjacent to the external opening. These folds fuse and divide based on their position. Around 9 weeks after conception, these external genitalia start to differentiate into a penis or a clitoris and labia majora. In males, testosterone directs the genital tubercle to elongate to become the penis. By 4 months after conception, the penis is mostly formed.16

In females, estrogen from the ovaries direct the genital tubercle to elongate only slightly. This forms the clitoris. Most of the female reproductive structures are correctly positioned by 12 weeks after conception.17 At this point, a doctor can usually tell if the baby is a boy or a girl by using an ultrasound.

How do teeth form?

Teeth start developing about 7 weeks after conception. They form from the interaction of cells near the early neural tube, called neural crest cells, and the external tissue layer, called ectoderm. Basically, the enamel coating on teeth comes from the ectoderm while the inner structure of a tooth, including the roots and dental pulp, comes from the neural crest cells.18 About 9 weeks after conception, the bud of a second tooth starts to form behind the first tooth. The second tooth will slowly develop into the permanent adult tooth. The first tooth to form becomes the baby tooth.19 Baby teeth usually emerge from the gums about 6 months after birth, and adult teeth replace the baby teeth throughout childhood.

Dive Deeper
Teeth start as buds in the seventh week after conception. Around nine weeks after conception, each bud splits...