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Fetal Development

The stages of fetal development, week by week

The term fetal development encompasses everything from fertilization (conception) until birth and can be divided into two main stages: the embryonic stage and the fetal stage.

The embryonic stage lasts from fertilization until the tenth week of gestation, when the fetal stage begins. This stage then ends at birth, which normally occurs between 37 and 40 weeks. It is important to remember that there is a distinction between embryonic age (the actual stage of development) and gestational age (the number of weeks since the last menstrual period). For this reason, stages of fetal development don't always correspond with stages of pregnancy, which often follow gestational age.

Fetal Development Week by Week

Fetal development begins at fertilization, which generally occurs about two weeks into gestation (15 to 21 days after the last menstrual period). Week 1 of embryonic development sees rapid growth from zygote (fertilized egg) to blastocyst. It may also mark the beginning of implantation. By the end of week 2, implantation is complete.

Week 3 sees the beginning of brain, heart, and vertebra development, while week 4 marks the beginning of a regular heartbeat and the development of several internal organs, as well as differentiation of forms that will eventually become the neck, head, arms, and legs. These continue to develop through weeks 5 and 6.

In week 7 of fetal development, the embryo is about three-quarters of an inch long and is beginning to develop joints and hair follicles. Its heartbeat can be heard using a fetal Doppler machine. This coincides with week 9 of gestation and marks the end of the embryonic stage.

Week 8 of fetal development, also known as week 10 of gestation, begins the fetal stage. At this point, the fetus is essentially formed, but structures must continue to develop and grow. Between now and week 10 of fetal development, tooth buds will form, the fetus will be able to form a fist, and the face becomes well formed.

Weeks 11 to 14 see the hardening of bones, the beginning of active movements, and the development of genitalia. After about week 13, sex prediction is extremely accurate.

At week 15, the fetus can sense light (although its eyelids are fused shut) and move its joints, and at weeks 16 and 17, it is growing finger- and toenails. Quickening generally happens around this time.

Between weeks 18 and 20, scalp hair becomes apparent, the fetus begins swallowing amniotic fluid and producing urine, and a startle response may be evident. In weeks 21 and 22, skin becomes opaque and eyelids and eyebrows grow.

At week 23, the fetus can hear, and at week 24, a fetus is officially viable.

Weeks 25 to 28 see development of the lungs -- by week 28, the fetus is capable of breathing (though it would still struggle if born at this stage).

At week 29, the head is in proportion. At week 30, the fetus weighs almost three pounds. At week 31, physical growth begins to slow, and at week 32, all five senses are working.

Weeks 33 and 34 see development of the brain (synapses formed) and immune system (antibodies transferred), and week 35 marks the beginning of the end, when the fetus begins to position itself for birth. Most babies born at 35 weeks survive without long-term problems.

Weeks 36 to 40 see continuing development of the lungs and further positioning in preparation for birth, which may occur at any time now but usually happens around week 37 or 38.

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