Characteristics of the Unborn Child
Growth and Development

Pregnancy begins at fertilization, or conception, with the union of a man’s sperm and a woman’s egg to form a single-cell embryo.  This brand new embryo contains the original copy of a new individual’s complete genetic code.  Gender, eye color, and other traits are determined at conception. 
Full-term pregnancy typically lasts 38 weeks from conception or 40 weeks from the first day of a woman’s last normal menstrual period (LMP). 
During pregnancy, physicians measure prenatal age in two ways. Most commonly, prenatal age is referenced from the first day of a woman’s last normal menstrual period (or LMP). Alternatively, prenatal age may be referenced from the time of conception, which typically occurs two weeks after a woman’s LMP. For example, six weeks after conception corresponds to eight weeks following LMP.

Some women may be unsure of the first day of the last menstrual period, so other methods can be used to determine the age of the fetus.  For example, ultrasound can be used to measure the head, abdomen, and thigh bone of the fetus, which then can help estimate prenatal age.
Most significant developmental milestones occur long before birth during the first eight weeks following conception when most body parts and all body systems appear and begin to function. 

The main divisions of the body, such as the head, chest, abdomen and pelvis, and arms and legs are established by about four weeks after conception.  Eight weeks after conception, except for the small size, the developing overall appearance and many internal structures resemble a newborn.

Many common daily activities seen in children and adults begin in the womb – starting more than 30 weeks before birth.  These activities include hiccups, touching the face, breathing motions, urination, right- or left-handedness, thumb sucking, swallowing, yawning, jaw movement, reflexes, REM sleep, hearing, taste, sensation, and so on.

The development of a fetus depends on many factors.  This booklet will discuss only normal growth and development.  The prenatal photos and drawings are not meant to convey precise size and/or detail, but rather the general shape and progression of  a fetus as it develops.
Unless otherwise noted, all prenatal ages in this publication are referenced from the start of the last normal menstrual period. In the following section titles, the corresponding prenatal ages referenced post-conception are presented is underneath the last menstrual period.

Conception

Shortly after a woman’s period begins, her body begins preparing for the possibility of pregnancy.

Approximately 2 weeks into her cycle, a woman releases an egg from one of her ovaries into her adjacent fallopian tube.  Conception is now possible for the next 24 hours or so. Conception signifies the beginning of pregnancy and the beginning of human development.

After conception, the single-cell embryo has a diameter of approximately 4 thousandths of an inch.