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Fetal Development Timeline: Weekly Development of the Fetus in the First Trimester of Pregnancy

Updated on July 6, 2015
Chris Telden profile image

Chris Telden knows how fortunate she is to be a mom at all, given her high-risk pregnancy and the challenges of extended breastfeeding.

One of the best parts of being pregnant is the ability to keep track of the stages of your baby's development throughout the duration of your pregnancy. When you learn how your baby is developing during the prenatal period, you begin the bonding process with your baby. The 40-week period of a normal pregnancy is divided into three trimesters, with the first trimester covering the first fourteen weeks. During the first trimester of pregnancy, your baby grows and develops at a stunningly fast rate. This prenatal timeline summarizes the week-by-week breakdown of the first trimester of fetal development, marking important milestones during this critical period of pregnancy.

Zygote--Embryo--Fetus--Oh, My!

Although you might hear people refer to the "fetus at 8 weeks" or "the fetus at 6 weeks," your baby at this point isn't called a fetus. At 1-2 weeks your baby's called a zygote, at about 3 weeks your baby's known as an embryo, and only by 10 weeks or so does your baby get the name of fetus.

Pregnancy First Trimester: Weeks 1-2

  • At the end of the first two weeks of pregnancy, which begins officially at the beginning of your last period, you won't notice any symptoms to tell you you're pregnant. This is because you're not, biologically.
  • In week 1 or week 2, the sperm fertilizes the egg and provides an X or Y chromosome to the zygote (the name for your baby at this point) which establishes your baby's gender.

What is a blastocyst?

Before implantation and about 5 days after fertilisation, a tiny embryo of just 150 cells forms when the zygote splits. This is the blastocyst. The baby at this point is simply a round sphere filled with fluid and cells.

Pregnancy First Trimester: Weeks 3-4

  • When the zygote (about 500 cells big, and counting) makes its way down the fallopian tube and arrives at the uterus, it becomes a blastocyst, which forms into an embryo as it attaches itself to the uterus.
  • At this time of implantation, you may experience some light spotting mimicking a very light menstrual period.
  • The beginnings of a nervous system and circulatory system develop in the embryo.
  • This week or the next, the placenta forms and begins to produce the pregnancy hormone, hCG.
  • Soon, a pregnancy test should be able to detect the presence of the pregnancy hormone in your urine to determine whether or not you're pregnant.

Pregnancy First Trimester: Weeks 5-7

  • Your baby's tiny heart begins to beat.
  • The umbilical cord forms.
  • Your baby's blood begins circulating through his system.
  • Leg and arm buds form and grow.
  • Nostrils open.
  • The lungs, brain, intestines, and pancreas make their appearance.
  • Week seven marks the arrival of elbows, feet, ears, eyes, nose, jaw, palate and teeth.

Pregnancy First Trimester: Weeks 8-10

  • Bone and cartilage develop and joints form.
  • Your baby's tongue makes its arrival.
  • The fetus's intestines migrate out of the umbilical cord into the abdominal cavity.
  • Nipples form and reproductive organs (ovaries or testes) develop.
  • Your baby begins to move, although you can't feel it. She might even be able to grab at an object with her newly sprung webbed fingers, or wiggle her webbed toes.
  • Around week ten, your baby officially graduates from “embryo” to “fetus” status.
  • She is top-heavy, with half her length (1.22 inches) being her head.
  • Each minute, her brain fires out 250,000 new neurons.
  • Now the placenta begins to do its stuff, and the most critical period of development is over.

Pregnancy First Trimester: Weeks 11-12

  • Your baby begins to look a lot more like a baby, with hair, separated fingers and toes, complete with fingernails and toenails, and external genitalia (which aren't yet visible on ultrasound).
  • Your baby's pancreas and liver begin to work, and the kidneys start to produce amniotic fluid.
  • Eyes, ears, and intestines shift closer to their ultimate destination.
  • Your baby gets a set of vocal cords.

Pregnancy First Trimester: Weeks 13-14

  • Your baby begins to go through the movements of breathing in practice for the real thing.
  • He may practice touching his hands together and may even suck his thumb.
  • Your baby's neck elongates.
  • Baby's eyebrows appear.
  • The prostate gland forms (or, if your baby's a girl, her ovaries shift to her pelvis from where they'd been in her abdomen).
  • The protective smattering of fine hair known as lanugo begins growing on your baby's skin.
  • By the end of the first trimester of pregnancy, you can hear your baby's heartbeat by Doppler.

At first trimester's end, your baby weighs a little over 40 grams, which is 1.5 ounces. Your baby has developed at all his critical systems at an amazing pace—and now is just under 3.5 inches long.

Brain Development Timeline

Your baby's brain development timeline begins just after he's conceived. By the time he's born, he'll have a full complement of neurons to last him all his life. That doesn't mean his brain will stop developing - on the contrary, although his brain will weigh in at an adult size when he's 14, his brain will continue to change over the course of his lifetime.


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