Pregnancy due dates are calculated by adding 40 weeks to the date of your last menstrual period. Technically, during the first two weeks of this time frame, most women are not yet pregnant. Fertilization occurs at week three when the sperm and egg unite to form a zygote in the fallopian tube. By week four, the zygote, now called a blastocyst, will make its way to the uterine wall and a placenta will begin to develop.
First Trimester
In the third week of pregnancy, which is the fifth week after conception, the blastocyst becomes an embryo. The embryo is less than 1/8-inch long. During week six, the embryo takes on a C-shape and the heart begins to pump blood. By the end of week eight, the embryo is about ½-inch long, and its fingers are beginning to form. The eyes, lip and nose are present. At around week 11, the embryo becomes a fetus and its genitals begin to develop. Week 12 marks the end of the first trimester. By this time, the fetus weighs about ½-ounce.
Second Trimester
By week 14, early bone tissue is developing, and you may be able to determine the fetus' gender via ultrasound. Fine hair known as lanugo develops. At week 16, the baby is capable of facial expression. By week 19, the baby can hear, and the mother may be able to feel a fluttering sensation in the abdomen. At week 20, the halfway point, the baby is covered in lanugo. You may be able to hear its heartbeat with a stethoscope. By the end of the second trimester, at week 27, the baby weighs about 2 pounds and may be viable with intensive medical care.
Third Trimester
The baby's eyes open at about week 28. At week 29, the skeletal structure is complete but bones will not harden, or ossify, until after birth. By week 32, lanugo disappears and the baby weighs a little less than 4 pounds. Starting at week 36, the baby will grow rapidly, gaining about half a pound each week. The baby is considered full-term at week 37. Week 40 is your due date, but it is common to deliver a baby a week early or late. The average baby weighs about 7 1/2 pounds at delivery.
Birth to 6 Months
The four basic types of infant development include social, linguistic, gross motor and fine motor skills. During the first month, the baby cries to express need. Most babies can smile during the second month. By the third month, the baby's repertoire of sound includes giggles, squeals and other non-verbal expressions. During the fourth month, most babies can laugh and sit upright with some support. The baby may begin teething during the fifth month. By the end of the sixth month, most babies can stand with assistance.
6 to 12 Months
Most babies can sit without support by the seventh month. Socially, infants may experience stranger shyness and separation anxiety. By 1 year old, many babies can say basic words made up of repeated syllables, such as "dada." Physically, 1-year-old babies can crawl, stand and walk a few steps with support.
References
- Mayo Clinic: Fetal Development --- The First Trimester
- Mayo Clinic: Fetal Development --- The Second Trimester
- MedlinePlus: Fetal Development
- Mayo Clinic: Fetal Development --- The Third Trimester
- March of Dimes: Developmental Milestones for Babies
- Centers for Disease Control: Important Milestones --- By the End of One Year
