An unborn baby is referred to as an embryo through the first eight weeks following fertilization; it is then referred to as a fetus until delivery.
Last menstrual period.
Implantation into the wall of the uterus
Brain, nervous system, spinal cord begin to form. Organogenesis begins: the formation and development of organs
Heart begins beating. External features begin to form
Eyes and ears begin to form; arm and leg buds begin to form; stomach forming; heart begins pumping blood through main vessels
Pancreas begins to form and secrete hormones. Eyelid forming and partially covers the eye.
Small nostrils visible. The embryo is about the size of a pencil eraser.
Sense of smell forming. Fingers and toes begin to lengthen.
Hearing and balance abilities present. Gender was determined at conception but individual parts are not yet discernible. Major joints like shoulders, knees, wrists, ankles, etc, move.
Fingers are separated. Head is rounding out and neck is becoming distinguishable. Yolk sac begins to disappear. The “embryo” becomes a “fetus”.
The fetus measures about 2 inches long. Eyelids fuse and won’t reopen again until week 28. Face is well formed. Bone beginning to harden.
Fingernails are forming.
Urine is being formed and discharged into the amniotic fluid. Arms and legs flex and kick.
Red blood cells are being formed in the liver. Fetus can “make faces” using his/her facial muscles.
Bones are continuing to harden and can be seen on ultrasound.
Average length is 4 ½ inches. Heart pumps about 25 quarts of blood each day. Sucking motions
can be made.
Fat begins to accumulate. Toenails are developing.
Sense of hearing is developing. Average length is 5 ½ inches.
Vernix, a protective cheesy-like substance, coats the skin. Hair may begin growing.
Fetal movement can be felt. Meconium, the substance seen in the first dirty diaper, is accumulating.
Baby can swallow. Eyelids and eyebrows are fully developed.1
A fine, downy, hair, called lanugo, covers the skin; this helps hold the vernix on the skin adding one more level of skin protection.
Skin is red and wrinkled. Rapid eye movements.
Regular sleep and wake patterns may be noticed. Average length is about 8 inches. Taste buds are developing.2
The startle reflex begins to develop.
Lungs are beginning to produce surfactant – a substance related to lung function. Average length is about 9 inches.
End of second trimester. Lungs and nervous system continue to mature.
Eyelids are partially reopened. Fetal weight gain starts to smooth out the wrinkled look. A 90%
survival rate if born early.[1. UNSW Embryology. (2015). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Main_Page]
Bones are fully developed. Fetal movement is stronger.
Eyes are often wide open. Average length is about 10 ½ inches, and average weight is about 3 pounds.3
Central nervous system is mature enough to control body temperature. Reproductive system development continues.
Toenails and fingernails fully formed. Fetal breathing movements may be noted. Vital minerals such as iron and calcium are being absorbed.
Baby can tell the difference between light and dark.
The lanugo covering is decreasing but the vernix covering is increasing. Average length is 12 inches.
Rapid weight gain – about a half pound per week – begins.
The head-down position is most common. Slightly decreased activity due to the crowded conditions inside the uterus.
Fetal organs are ready to function on their own.
Baby’s hand can firmly grasp. Average weight is about 6 ½ pounds.
Antibodies are being formed by the placenta. Chest is more prominent. Fat stores increasing.
Approximate due date.
- Baby Center.Retrieved from www.babycenter.com/fetal-development-images Retrieved Sept 25, 2015 ↩
- Mayo Clinic. (n.d.). Healthy Lifestyle-Mayo Clinic. Retrieved from http://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development ↩
- U.S. National Library of Medicine. (2013, September 30). Fetal development: MedlinePlus Medical Encyclopedia. Retrieved from http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm ↩