Pregnancy is considered 40 weeks long, beginning from the first day of your last menstrual period to 40 weeks later. A 28-day cycle is the most common method used to determine whether a baby is developing at the rate that it should progress. Though a little confusing, doctors will tell you that you have already been pregnant for two weeks at the time of your conception. This is because the sperm and egg join two weeks after the first day of your last period.
During the nine months of pregnancy, most babies generally develop at the same rate. This is often referred to as fetal development, with a woman’s due date set at exactly 40 weeks. A baby born prior to the 37th week is considered premature.
The first trimester is considered as the first third of your pregnancy, or week 1 through week 13. In the beginning, there are so many amazing things happening in your body that it is hard not to notice or feel a physical difference. The baby — commonly called an embryo this early — begins to form from a single fertilized egg during week 2, dividing into two cells, then four, then eight, and so on. The embryo floats from the fallopian tube down into the uterus, where it implants itself. During week 3, the developing embryo is only one-hundredth of an inch long, but it is rapidly multiplying cells until week 4, where a woman may first suspect that she’s pregnant due to her first missed period.
It is during week 4 that the embryo’s cells begin to take on specific functions; the backbone, spinal column, and nervous system are forming. At 5 weeks and only the size of a raisin, the tiny heart begins to beat and the baby’s main external features like ears start to form. At 6 weeks, the kidneys, liver, and most organs have already started to develop and arm and leg buds become visible. Fingers start to take shape at 7 weeks, when the heart also forms into right and left chambers; at 8 weeks the unborn baby is considered a fetus and is about half-an-inch long. At 10 weeks the fetus’ heart is nearly completely developed, and genitals have begun to form. Vital organs are almost fully formed at 11 weeks, and vocal chords let a baby cry silently at 12 weeks, when the baby’s brain is formed and it can feel pain.
Weeks 13 through 26 are the second trimester, often considered the easiest during pregnancy.
It is during this time that most nausea or morning sickness subsides, fatigue lessens, sleeping comes easier, and pregnant women generally have an overall happier three months. During the second trimester, or fourth, fifth and sixth months, the baby’s fingerprints are in place, its facial features start to resemble those of an adult, the taste buds form and a baby can taste its mother’s meals. Around week 16 is when a pregnant woman can expect to feel the baby kick or move for the first time, as it can now grasp with its hands and do somersaults. Lungs begin to inhale amniotic fluid around week 17, and it’s between week 18 and week 21 that you will be able to tell the baby’s gender — that is, if it cooperates during an ultrasound! It must be in a good position to determine if a fetus has labia or a penis. A child can hear and recognize its mother’s voice around 20 weeks, which is an important time for sensory development like smelling, tasting, seeing, hearing, and touching. And although it is tiny and fragile, weighing less than a pound, a baby born at 20 weeks could possibly live with medical intervention.
It is at week 21 when baby begins to fatten up, gaining fat steadily in order to stay warm. At week 23, the fetus is in exact proportion to a newborn except that he or she is a thinner version since it hasn’t gained enough body fat yet. Baby can practice breathing some more during week 25, when it weighs over 1-1/4 pounds and is built well proportionally. Its brain is growing at a rapid speed, and the baby is also growing swiftly, filling the space inside your uterus. During week 26, a baby makes breathing movements without air in its lungs, and a fetal brain scan will show a response to touch.
Week 27 begins the third trimester, when a fetus does most of its growing. During these last three months, baby’s eyes begin to open, it can suck its thumb, its brain and head grows bigger, its ears start to increasingly hear sounds, and its lungs and kidneys mature. By week 38, a pregnancy is full-term and babies weigh an average of 7.5 pounds and are 18 to 20 inches long.