First Trimester Part 1
What is the first trimester?
The way we measure the length of a pregnancy and its stages can be very confusing. A pregnancy lasts about 280 days, 40 weeks, or roughly 9 months. People count pregnancies in months, while healthcare providers count pregnancies in weeks. We calculate a pregnancy from the first day of a woman’s last normal menstrual period until 40 weeks, which we assign as the due date. Conception usually happens around the time of ovulation, which occurs about two weeks after the first day of your menstrual period. Therefore, the actual duration of a pregnancy is 38 weeks. This is assuming that a woman delivers her baby on the due date, but only 3 % of women deliver exactly on their assigned due date. It is standard of care in the United States to not allow a pregnancy go beyond 42 weeks gestation. Therefore, the longest a woman can be pregnant in the United States is 40 weeks. If we divide the potential 42 weeks from the first day of the last menstrual period to 42 weeks into trimesters, we get three 14 week periods. The first trimester of pregnancy is the time from the last menstrual period until 13 weeks and 6 days of gestation.
How important is the first trimester?
The first trimester is the most critical stage of a pregnancy. Rapid changes during early development of the baby occur during the first 14 weeks of pregnancy. Babies are most susceptible to potential harmful substances and environment conditions during this important stage.
What’s happening with the baby in the first trimester?
The first trimester is divided into two development stages. The first stage, referred to as the embryonic stage, last seven weeks. The second stage goes beyond the first trimester into birth and is called the fetal stage.
By 4 weeks gestation the embryo’s ears, nose, eyes, spine, digestive tube and nervous systems are forming. The tubal structure that will lead to a heart also starts beating. There is definitely a life formed in the womb, most times before you realize that you are pregnant. By 8 weeks gestation, the fetus has formed all organs that a baby will have at birth. The heart is fully functional and bones begin to form. By 12 weeks gestation, the fetus teeth buds appear. Fingernails and toenails are forming. Kidneys start making urine. Ureters connect the kidneys to the bladder, which is starting to fill up with urine. Urine secreted into the womb makes up amniotic fluid. External genitalia begins to form and by 14 weeks in most fetuses we can distinguish between boy and girl parts. By 12 weeks the fetus is floating in amniotic fluid and can start moving. You don’t start feeling movement until around 17 to 18 weeks gestation because of the size of the fetus and the position. At 12 weeks the uterus is still within the pelvis. Fetal heart tones can be detected by your doctor around as early as 10 weeks, but definitely by 12 weeks gestation.