Pregnancy Trimesters Growth

The first 13 weeks of pregnancy represent the most critical period of development for the fetus. During this time, the fetus is most susceptible to effects from outside influences, such as medications you take or other substances you ingest or are exposed to.

By the end of the first half of your pregnancy-20 weeks-your baby has developed nearly all of its organs. During the last half of pregnancy, these organs mature, and your baby grows and gains weight, until it is ready to be born.

Your baby goes through an early period of development, called embryonic development, which lasts from fertilization through week 8. The fetal development period lasts from week 9 until your baby is born.

TRIMESTERS OF PREGNANCY
During the first 8 weeks of pregnancy, your baby is called an embryo. During the rest of your pregnancy, the developing baby is referred to as a fetus.

Your pregnancy is divided into 13-week periods called trimesters.
The 1st trimester is the one of greatest change for the developing baby. During this time, your baby grows from a collection of cells the size of the head of a pin to a fetus the size of a grapefruit. Organs begin to develop, and your baby begins to look like a baby.

At the beginning of the 2nd trimester, your baby weighs less than an ounce (25 g) and is only about 4 inches (110 cm) long. By the beginning of the 3rd trimester, he is almost 16 inches (34 cm) long and weighs more than 2 pounds (1,000 g). At delivery, your baby will weigh close to 7 liz pounds (3400 g) and be about 21 liz inches (48 cm) long.

Very few, if any, structures in the fetus are formed after the 12th week of pregnancy. This means your baby has formed all of its major organ systems by the end of the 1 st trimester. However, these structures continue to grow until your baby is born.

If a baby is born before the 38th week of pregnancy, he is called a preterm baby. An infant born between the 38th and 42nd week of pregnancy is called a term baby or a full-term infant. A baby delivered after 42 weeks of pregnancy is called a post term baby.