Note: Information provided on this page is for reference only,
please seek medical assistance when in doubt
Pregnancy is typically broken into three periods, or trimesters, each of about three months
or about 14 weeks, resulting in a total duration of 42 weeks, although the average duration of
pregnancy is actually about 40 weeks.
Usually after 14 days of the last normal menstrual period ovulation occurs where the egg
cell is released from one of the female's two ovaries. This egg cell unites with male sperm in one
of the two fallopian tubes; this process is referred as fertilization where the egg cell fuses
with the male gamete. The fertilized egg, known as a zygote, then moves toward the uterus, a
journey that can take up to a week to complete. Cell division begins approximately 24 to 36
hours after the male and female cells unite. Cell division continues at a rapid rate and the
cells then develop into what is known as a blastocyst. The blastocyst arrives at the uterus
and attaches to the uterine wall, a process known as implantation.
In first 10 weeks of gestation the cell divides rapidly and begin to differentiate into
the various body systems, the basic outlines of the organ, body, and nervous systems are
established. By the end of the embryonic stage, the beginnings of features such as fingers,
eyes, mouth, and ears become visible, during this time, there is also development of structures
important to support the embryo, including the placenta and umbilical cord. Placenta
connects the developing embryo to the uterine wall to allow nutrient uptake, waste elimination,
and gas exchange via the mother's blood supply while the umbilical cord is the connecting cord
from the embryo or fetus to the placenta.
After about 10 weeks of gestational age, the embryo becomes known as a fetus when the
risk of miscarriage decreases sharply, at this stage the length of a fetus is typically about
30 mm (1.2 inches) and the heart can be seen beating via ultrasound; the fetus can be seen
making various involuntary motions at this stage. During continued fetal development, the
early body systems and structures that were established in the embryonic stage continue to
develop. Sex organs begin to appear during the third month of gestation. The fetus continues
to grow in both weight and length, although the majority of the physical growth occurs only in
the last weeks of pregnancy.