Most women and many pregnancy books will refer to pregnancy by months. Seems to make sense—3 trimesters, 3 months per trimester. Then why do doctors measure pregnancy in weeks? The answer lies in how a pregnancy is dated and how it develops.
So how do doctors determine your due date? A due date is based on the first day of a woman's last menstrual period. This is a relatively reliable and obvious date that most women remember with accuracy. In many instances, an early first-trimester ultrasound can be ordered to confirm dating or to obtain a due date if the last menstrual period is uncertain or unknown.
Obstetricians use weeks instead of months because not all months are created equal. Simply put, most months contain four and a half weeks. If a woman says she is 6 months pregnant, she could be anywhere between 24 and 27 weeks along. This matters if there are pregnancy complications that may require intervention.
It's important to know exactly how far along a pregnancy is because if medical issues develop and earlier delivery needs to be considered, interventions for the mother and the baby's well-being will differ based on how many weeks of development have already occurred.
Understanding weeks of development
Roughly 2 weeks after your period, you will ovulate, and if all the stars align, you will become pregnant. Congratulations! The fertilized egg now begins the journey through your fallopian tube and into your uterus, where it will implant and begin developing. By week 4, organs are already beginning to form. The first trimester is one of the most critical times for development, which is why it is important to take a prenatal vitamin if you are not using a form of birth control. Many organ systems are already beginning to form prior to knowledge of the pregnancy.
By 6 to 8 weeks, you may begin to notice symptoms, such as nausea or breast tenderness. You may have missed a period and taken a pregnancy test. By week 10 of pregnancy, most of the baby's major organ systems have been formed, though they will continue to develop throughout the pregnancy.
At 14 weeks, you will be entering the second trimester and will typically start to feel better. Energy levels improve, nausea resolves and the baby starts to get larger. By 18 to 22 weeks of pregnancy, you may start to feel the baby's movements, which is an exciting milestone.
At around 20 weeks, an ultrasound will be performed to look at the baby from head to toe. This is known as an anatomy survey, and is done to make sure all of the organs have developed normally. This is also when you can find out the baby's gender.
If a woman says she is 6 months pregnant, she could be anywhere between 24 and 27 weeks along. This matters if there are pregnancy complications that may require intervention.
By the 24th week, the baby is beginning to gain weight. The lungs are beginning to mature, and at this gestational age, the baby is considered "viable." This means that if the baby is born prematurely, they have about a 50 percent chance of survival with neonatal intensive care support, although they may have significant and possibly lifelong health challenges.
Between 26 and 28 weeks, you will be given a glucose tolerance test to rule out gestational diabetes, which all pregnant women are at risk for due to hormones produced by the placenta. The baby's lungs will continue to develop and mature. By 28 weeks, the baby has an improved chance of survival if born, though they are still considered quite premature. Every week counts!
At 37 weeks of pregnancy, the baby is considered full-term. From this point on, the baby will continue to gain weight and further develop their lung and brain functions. If labor begins spontaneously during this time frame, there is no reason to stop it, but labor cannot be induced prior to 39 weeks without a medical reason.
Once you reach 40 weeks, you've made it to your due date—but you aren't in labor. What gives? Well, only 5 percent of babies are actually born on their due date. Again, the due date is based on that last menstrual period and gives us a date at which we expect the pregnancy to be fully developed.
Most doctors will recommend that a healthy, low-risk pregnancy be delivered prior to 42 weeks of gestation because the placenta (the organ that feeds and oxygenates the baby inside the uterus) is not designed to last forever. It typically stops functioning well enough to safely support the baby at that time, so the risks involved in remaining pregnant are greater than the risk of inducing labor.
If your pregnancy goes beyond the due date, additional testing may be recommended to ensure the baby stays healthy. If you do not go into labor on your own, your doctor or midwife may schedule an induction to start your labor with medication.