Week 1 (0 to 7 days)

Day 1

  • Conception: dad’s sperm penetrates a mom’s oocyte (egg) in the fallopian tube. A new, unique, never-to-be-repeated human being begins the first day of life as a single cell embryo (zygote).
  • Baby is smaller than a grain of sugar, but a complete DNA / genetic blueprint is already present for all the tiny person will become. All inherited physical features are already set – eye, hair and skin color, fingerprints, gender, height, shape of the face and body, dimples, elements of personality, various intelligences, etc.

Days 2-5

  • Baby’s cells continually divide. Each will perform different functions – circulatory, muscular, neural, skeletal, etc.
  • Baby is traveling down the fallopian tube toward the uterus to find the food and shelter needed to grow and develop.

Week 2 (8 to 14 days)

Day 8

  • Baby is about the same size as the period at the end of this sentence and growing exponentially.
  • Implantation is completed – baby snuggles into the wall of the mom’s womb and begins to draw nourishment.

Doctors can detect pregnancy as early as 8 or 9 days after fertilization. Most moms, however, have no idea they are pregnant for several weeks. (It is at this point that mom misses her first period, but often she may experience “implantation bleeding” which can mimic a light period.) Sensitive pregnancy tests can be positive, but it depends on the level of hormone (hCG) produced by the baby. Usually by the time mom discovers she is pregnant, many important structures in the growing embryo have long since developed.

Week 3 (15 to 21 days)

Day 17

  • Baby’s sex cells that will become sperm / eggs for creating grandchildren begin to group together.

Day 20

  • Foundations of baby’s brain, spinal cord, heart and central nervous system are being established.

Day 21

  • Baby’s heart has begun to beat – first organ to function, no bigger than a poppy seed, initially beating at 70 times per minute, and visible by ultrasound almost immediately.
  • Blood vessels begin to form.
  • Eyes and ears begin to develop.

MILESTONES

21
Baby’s heart has begun to beat.
Egg and sperm Credit: Spike Walker. Wellcome Images

Egg and sperm
Credit: Spike Walker. Wellcome Images

Human blastocyst hatching out of zona Credit: Yorgos Nikas. Wellcome Images http://wellcomeimages.org

Human blastocyst hatching out of zona
Credit: Yorgos Nikas. Wellcome Images

Human 16 cell embryo Credit: M. Johnson. Wellcome Images images@wellcome.ac.uk http://wellcomeimages.org A human sixteen-cell embryo, also called a morula (latin for mulberry). The embryo has gone through four cycles of cell division to get to this stage and the cells have undergone compaction whereby they flatten onto each other. Copyrighted work available under Creative Commons by-nc-nd 2.0 UK, see http://wellcomeimages.org/indexplus/page/Prices.html

Human 16 cell embryo
Credit: M. Johnson. Wellcome Images
A human sixteen-cell embryo, also called a morula
(latin for mulberry). The embryo has gone through
four cycles of cell division to get to this stage
and the cells have undergone compaction whereby
they flatten onto each other.