Pregnancy Trimester Guide

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When a woman becomes pregnant, her body goes through changes beginning at the moment of conception. The average pregnancy lasts 40 weeks (280 days) and is broken into three trimesters. The estimated due date is calculated from the first day of the last menstrual period. Most women do not deliver on their due date, instead delivering anywhere from three weeks early to two weeks late.

Pregnancy health is very important and a prenatal multivitamin helps ensure this as well as preventing many birth defects.

First Trimester

The first trimester constitutes the first 12 weeks of pregnancy. Many changes happen in the body, including morning sickness. When the sperm first meets the egg, they form a zygote (single cell), divide into a blastocyst (multiple cells) and travel through the fallopian tube into the uterus. Approximately five days later, the blastocyst implants in the uterus and becomes an embryo and placenta. The baby grows inside the amniotic sac within the uterus. The embryo becomes a fetus after eight weeks, forming arms, legs, fingers and toes. The brain enlarges, eyes form and external genitalia emerges by the 12th week.

Second Trimester

The second trimester constitutes weeks 13 through 26. Morning sickness generally subsides, baby movements can begin to be felt and the woman begins to show. The baby’s head becomes proportionate to the body, bones solidify and by 26 weeks the baby looks much more like a human baby. The baby can hear and swallow. Lung development increases between 20 and 25 weeks. Between 26 and 28 weeks, the baby’s eyes open and hair appears on the head and body. The central nervous system matures as well. The baby becomes viable (meaning if it were to be born it would have a chance of surviving) around 23 to 24 weeks.
Third Trimester

The third trimester constitutes the remainder of the pregnancy. The fetus spends most of the third trimester growing and developing organs. The baby spends the last two months preparing for birth. The changes are not as dramatic as they were during the first two trimesters. The fetus assumes vertex presentation (head-down position) around 28 to 34 weeks. Breech presentation (buttocks-down position) occurs in a small percentage of pregnancies, usually requiring a Caesarean section. Amniotic fluid volume reaches its maximum level by 36 weeks. Braxton-Hicks contractions (contractions that do not mean labor) can occur in the late second trimester to early third trimester. The baby drops lower into the pelvis a month or so before delivery. Women who have had previous children may not drop until labor starts.


* Pregnancy for Dummies, 3rd Edition; Joanne Stone, MD Keith Eddleman, MD Mary Duenwald; 2009

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