Mount Auburn Hospital - Great Expectations

How we count the “weeks of pregnancy” can seem confusing. A normal pregnancy lasts 40 weeks. If we say each month has exactly 4 seven-day weeks, then a pregnancy lasts 10 months, not 9. The total number of days is about the same though. During every month of your pregnancy, your baby continues to grow, develop, and change.

HOW BABIES GROW AND CHANGE

• Pregnancy begins when the egg and sperm come together in the fallopian tube • Over a few days, the fertilized egg travels down the tube and implants in the uterus • The ball of cells that make up the fertilized egg begins to divide and specialize • The placenta, amniotic fluid, and embryo begin to form • This process takes about 4 weeks from the first day of your last period • Circulatory system and other vital organs have begun to form • This includes the baby’s heart, brain, spinal cord, lungs, eyes, and ears • The placenta and umbilical cord develop to carry oxygen and nutrients to the embryo • The baby is now protected from harm in a sack of liquid called amniotic fluid • By the end of this month, all major organs and body systems have started to develop • The embryo is now called a fetus and is beginning to look more like a human • Arms and legs start to form, and elbows take shape; the baby starts bending and flexing their arms • Fingers and toes are growing, and facial features are becoming more obvious • The bones, muscles, and intestines are also developing • The baby’s head seems huge because the brain is growing very quickly • Fingernails and toenails are growing, and some hair may have sprouted • Eyelids are fused shut, but eyelashes and eyebrows have begun to form • The kidneys have begun working and now produce urine • For the first time, the baby’s heartbeat can be heard by doppler • Your baby is moving, swallowing, and listening • The baby’s teeth, eyelids, eyelashes, and limbs are developing in detail and moving into position • Hands are becoming more functional; the baby may play with their fist or suck their thumb • The baby’s neck is getting longer, and the chin is not resting on the chest anymore • Very fine hair (lanugo) and a waxy substance (vernix) cover the baby’s body • Lanugo and vernix protect the baby’s skin while the baby floats in amniotic fluid • You may feel the baby move for the first time around 16-20 weeks

SIZE About 0.039 inches MONTH [WEEKS 1-4] 1

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MONTH [WEEKS 5-8] 2

SIZE About ½ inch

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MONTH [WEEKS 9-12] 3

SIZE About 2" long

WEIGHT Less than 1 ounce

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MONTH [WEEKS 13-16] 4

SIZE About 4" long

WEIGHT About 3½ ounces

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MONTH [WEEKS 17-20] 5

SIZE More than 6" long

WEIGHT About 11 ounces

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36 Your Guide to A Healthy Pregnancy