Fetal Development

Gestational age is defined in weeks and represents the end of that week or "completed weeks".  Conception actually occurs at the end of the second week (week 2 - or 2 completed weeks).  Pregnancies are dated from the first day of the last menstrual period because we often do not know the day of conception.  Because of this, the gestational age includes the two weeks before the pregnancy actually begins.

Week 1: This is the time the uterus is getting ready for a pregnancy to occur.

Week 2: Conception occurs when a sperm unites with an ovum (egg).  This new cell, called a zygote, is the beginning of a new, unique individual.  It is a large cell (compared to others) but is just visible to the naked eye as a tiny speck.  It will divide and travel through the Fallopian to the uterus over the next two weeks.

Week 3: The developing embryo has divided into multiple cells and is entering the uterus where it will "implant".  These cells have already begun to form layers that are the early digestive system, organs, and skin.

Week 4: The complex interaction between the developing embryo and the mother has begun as tissue surrounding the embryo implants into the lining of the mother's uterus (and begins to form the placenta).  The hormones produced cause the mother to skip that menstrual cycle and are detected with a pregnancy test.

Week 5: The embryo is forming the beginnings of nervous system and muscular and skeletal system.  Blood and blood vessels are forming and a primitive heart is present.

Week 6: At the beginning of this week (only 22 days after conception) the embryo's tiny heart begins to beat.  The embryo folds from an almost straight disc into its 3-dimensional form.  Arm and leg buds appear, facial structures begin to develop, and the brain begins to grow.

Week 7: Arm and leg buds form into "paddle like" structures then begin to form fingers and toes.  Mouth, ears and eyes are forming.  Its length has doubled in the last week.

3D Ultrasound of a 6 Week Old Embryo

Week 8: The embryo has fingers and toes but they are still webbed . Elbows and knees are visible.  An upper lip is present.  The brain is growing rapidly and the skin overlying the brain is less transparent.  The heart is more fully developed and beats at about 160 beats per minute (about twice as fast as an adult).

Week 9: Eyelids and ears are more developed and the tip of the nose is recognizable.  Arms and legs are longer and bent at the elbows and knees.  Internal organs are present and protrude out of the belly wall.

Week 10: This marks the beginning of the fetal period.  All of the essential internal and external structures are present in their beginning forms.  The fingers and toes are completely separated and bones begin to form.  Male or female parts begin to differentiate.  The fetus is 3 cm long at this point.

11-14 Weeks: The fetus will double in size during this period and begins to react to stimuli (such as sucking when lips are stroked).  Male and female parts look different and fingernails and toenails are present.

15-18 Weeks: Growth is very rapid during these weeks.  At 16 weeks it is 4-5 inches long and about 3 ounces.  It makes facial expressions, can have hiccups and can make a fist.  It is growing eyelids and scalp hair (in its own characteristic pattern) and fat stores are laid down under the skin.  By 18 weeks it can react to light and begins to hear.

19-22 Weeks: Most women will feel the kicks and movements of the fetus by this time.  The skin becomes covered with a cheese-like protective coating called vernix over down-like hair called lanugo.  External male and female organs are visible by ultrasound.   The fetus is almost 1 pound by 22 weeks.

23-28 Weeks: The fetus will add substantial weight during this period - to 2-3 pounds by 28 weeks.  The skin becomes less wrinkled as more fat forms under the skin.  They will open and close their eyes and begin making surfactant in the lungs to help them breathe after birth.  They can survive if born this early (from about 10% at 23 weeks to over 90% at 28 weeks.

29-36 Weeks: The fetus will add about 1/2 pound each week and movements become harder and stronger.  It will sleep 20-40 minutes at a time between active periods.  Downy "lanugo" hair falls off and the cheese-like "vernix" gets thicker.  The fat pads in the cheeks get larger so it can suck better after birth.

37 Weeks: the fetus is now full term and can be born anytime between 37 and 41-42 weeks.