Prenatal Development

Before Fertilization:

• During ovulation, an egg (a living human cell, also called an ovum) is released from a woman’s ovary into one of the fallopian tubes. If it remains unfertilized, it will die within 24 hours.
• Several hundred million sperm (also living human cells) are released during ejaculation. Sperm can live for five to seven days within a woman’s body. The sperm travel from the vagina through the cervix and uterus to the fallopian tube containing the egg. Some women may release an egg into each fallopian tube, which can result in non-identical twins.

At Fertilization:

• A single sperm succeeds in penetrating the egg. About 20 hours later the nucleus of the sperm fuses with the nucleus of the egg, forming a single cell called a zygote.
• The fertilized human egg has the same basic cellular chemistry as all other living things.
• Soon after fertilization, the zygote travels down the fallopian tube toward the uterus.
• Although rare, it is possible for a fertilized egg to begin growing in the fallopian tube. This is referred to as an ectopic (tubal) pregnancy. This is a life threatening condition and medication is needed in order to terminate the pregnancy.

At 2 Weeks' Gestation (from Conception):

• Gestation simply means a period of development. At two weeks gestation, a change in the zygote occurs which ends its capability of splitting into (identical) twins: "singularity" is achieved. The zygote is now called an embryo.
• Pregnancy (with its hormonal changes) begins when and if the zygote implants in the woman's uterus.

At 4 Weeks' Gestation (from Conception):

• The human embryo has a tail and looks very similar to embryos of other vertebrates, from fish to mammals. It is about half a centimetre in size.
• A primitive heart which generates its own electrical rhythm is located outside the body, allowing the growing organism to circulate nutrients and waste products.
• Like many mammals, human embryos begin without gender.
• The brain, spinal cord, heart, and other organs begin to form.

At 8 Weeks' Gestation (from Conception):

• The embryo is called a fetus anytime after 8 weeks gestation.
• It has the primitive beginnings of most of the major body organs. Organ functions have not yet developed.
• The heart is beginning to form into a four-chambered structure.
• The fetus moves by reflex.
• No brain waves (regular electrical patterns) exist yet.
• The eyes (still developing) start to move from the sides of the head toward the front, and the tail has disappeared.
• The fetus has begun to look human, with a rudimentary face, limbs, hands and feet (webbing disappears).
• The umbilical cord is clearly visible.
• It weighs approximately one gram and is about 3cm long.

At 12 Weeks' Gestation (from Conception):

• The second trimester of pregnancy has begun.
• External genital organs begin to differentiate.
• Reflex movements continue to occur.
• The fetus weighs about 28g.

At 16 Weeks' Gestation:

• The organs continue to grow and differentiate.
• The pregnant woman may begin to feel the fetus' movements around this time.
• The body of the fetus grows dramatically. It weighs about 170g.

At 20 Weeks' Gestation:

• The rate of fetal growth slows down.
• The internal organs continue to mature, although the lungs remain immature.
• The eyelids are completely fused.
• The fetus responds to noise and movements increase.

At 24 Weeks' Gestation:

• This is the beginning of the third trimester of pregnancy.
• The earliest point at which a fetus can potentially survive outside the womb is about 23-24 weeks of gestation (500g).
• The fetus is not viable until this point (23-24 weeks) because the fetal lungs are not mature enough to permit mechanically-assisted breathing.

At 28 Weeks' Gestation:

• The eyes are now open.
• The fetus begins to show signs of either being awake or sleeping.
• This is when the onset of cognitive awareness (brain life) occurs.
• The lungs are continuing to mature, increasing the chance of survival in the event of premature delivery.

At 32 Weeks' Gestation:

• The fetus continues to grow and gains an increased amount of fat.
• If born, the fetus has a high chance of survival.

Birth (~38 Weeks' Gestation):

• Birth is a multi-stage process, ending with the baby existing independent of its mother.  

Note: it is estimated that approximately 1 out of 4 pregnancies end in a miscarriage.

1. Healthwise. 2005. “Miscarriage – Topic Overview.” BC Health Guide. Retrieved November 26, 2007. <http://www.bchealthguide.org/kbase/topic/mini/hw44090/overview.htm>