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Your pregnancy countdown can get jumbled pretty quickly. Are you 13 weeks pregnant or in week 14? First trimester or your second? Three months pregnant or four?
Fortunately, the answers are (relatively) simple once you know what to count. Here’s what you need to know about how many weeks, months and trimesters there are in a pregnancy — and where you fit in.
How many weeks are in a pregnancy?
There are 40 weeks (or 280 days) in a pregnancy. Your due date is calculated starting from the first day of your last menstrual period (even though most pregnancies are not conceived until two weeks later). Keep in mind that only about 30 percent of pregnancies will reach precisely week 40.
How many weeks are in a trimester?
There are 13 or 14 weeks per trimester. (Just divide 40 by three and you’ll see why the breakdown isn’t exact.)
How many trimesters are in a pregnancy?
There are three trimesters in a pregnancy: the first trimester, which lasts from weeks 1 to 13, the second trimester, which lasts from week 14 to week 27, and the third trimester, which lasts from week 28 to week 40 (and up).
Is pregnancy nine or 10 months long?
Your 40 weeks of pregnancy are counted as nine months. But wait … there are four weeks in a month, which would make 40 weeks 10 months. Right? Not exactly.
Four weeks is 28 days, but months (with the exception of February, of course) have 30 or 31 days, making each month about 4.3 weeks long.
How many weeks is a full-term pregnancy?
If you're carrying a single baby, a full-term pregnancy is between the start of 39 weeks up to the end of 40 weeks. If you're carrying twins, a full-term pregnancy is considered at least 38 weeks.
The American College of Obstetricians and Gynecologists (ACOG) uses the following terms for pregnancy, depending on when your baby is born:
- Preterm: 20 weeks to 37 weeks
- Early term: 37 weeks to 38 weeks 6 days
- Full term: 39 weeks to 40 weeks 6 days
- Late term: 41 weeks to 41 weeks 6 days
- Post term: after 42 weeks
Pregnancy months to weeks chart
Here’s a simple breakdown of approximately how the weeks, months and trimesters are counted in pregnancy:
Trimester 1
- Month 1: Weeks 1 to 4
- Month 2: Weeks 5 to 8
- Month 3: Weeks 9 to 13
Trimester 2
- Month 4: Weeks 14 to 17
- Month 5: Weeks 18 to 22
- Month 6: Weeks 23 to 27
Trimester 3
- Month 7: Weeks 28 to 31
- Month 8: Weeks 32 to 35
- Month 9: Weeks 36 to 40
How do I know what week I am currently in my pregnancy?
Your doctor says you're 15 weeks pregnant, but you’re also hearing that you’re in week 16. Which is right? The short answer: both.
It turns out how many weeks into pregnancy you are is a little different than how many weeks pregnant you are. If you’re in week 16, you’re actually 15 weeks and some days pregnant. Fifteen full weeks have gone by, but not 16 weeks, so people say that you're both 15 weeks pregnant and in week 16 of pregnancy.
To clear things up, think about birthdays. When you turned 1 year old, you had lived through your first year and were starting on your second. In other words, on your first birthday, you were in day one of your second year. But no one said you were 2 years old until you'd finished that second year and had begun your third.
The same rules apply when talking about your weeks of pregnancy: You are x weeks pregnant, but in your x+1 week of pregnancy.
Wondering how your practitioner estimated your due date? It’s calculated counting from the first day of your last menstrual period (LMP), not the day you conceived. So try to remember the first day of your last period and add 40 weeks to that.
You can also calculate your due date with the following formula:
- Subtract three months from the first day of your last menstrual period
- Add seven days
- Add one year
For example, if the first day of your last menstrual period was June 2, 2022, subtract three months to get March 2, 2022. Add seven days and you'll get March 9, 2022. Finally, add one year to find your estimated due date of March 9, 2023.
Why is pregnancy calculated from your last period?
Why start the clock on pregnancy before sperm even meets egg (and, actually, before your ovary even dropped the fated egg that made your baby)? Using your last period is simply a more reliable benchmark to date a pregnancy from. Even if you’re a master of cervical mucus and are sure about the day you ovulated as well as the day(s) you had sex, you probably can’t pinpoint the exact moment of conception.
That’s because there’s a bigger window than you might think: Sperm can hang out and wait for an egg to fertilize for three to five days after they’ve arrived, and an egg can be fertilized for up to 24 hours after it’s been released. That's also good news, since it means you’ll already have clocked four weeks of pregnancy by the time you miss your period.
- What to Expect When You're Expecting, 5th edition, Heidi Murkoff.
- What to Expect Before You're Expecting, 2nd edition, Heidi Murkoff.
- WhatToExpect.com, Cervical Mucus and Pregnancy, April 2022.
- WhatToExpect.com, Due Date Calculator, November 2021.
- American College of Obstetricians and Gynecologists, How Your Fetus Grows During Pregnancy, December 2021.
- American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning, November 2020.
- American College of Obstetricians and Gynecologists, When Pregnancy Goes Past Your Due Date, November 2021.
- American College of Obstetricians and Gynecologists, Preterm Labor and Birth, November 2021.
- American College of Obstetricians and Gynecologists, Definition of Term Pregnancy, 2017.
- American College of Obstetricians and Gynecologists, Methods for Estimating the Due Date, May 2017.
- American College of Obstetricians and Gynecologists, Multiple Pregnancy, February 2021.
- March of Dimes, What Is Full Term?, September 2018.
- Mayo Clinic, How to Get Pregnant, December 2021.
- National Institutes of Health, National Library of Medicine, Length of Human Pregnancy and Contributors to Its Natural Variation, October 2013.
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The first trimester, lasting one to 13 weeks, begins at the start of your last period and ends after the thirteenth week of pregnancy. If implantation occurs, you can often receive a positive pregnancy test result the day of your missed period, or about 14 days after conception. However, pregnancy hormone human chorionic gonadotropin (hCG) can sometimes be detected as early as 10 days after conception.
Once you receive a positive pregnancy test result, it’s important to schedule an appointment with your OBGYN. Around seven or eight weeks, your doctor will likely meet with you for an exam, ultrasound and variety of prenatal tests. These tests may include blood tests, urine tests and genetic screenings. Your doctor will also review your personal and family medical history to be aware of any possible genetic conditions. This initial visit will establish your estimated due date as well.
“Accurate dating is very important in a pregnancy,” says Dr. Vernon. “It allows the practitioner the opportunity to confidently schedule time-sensitive tests, such as a nuchal translucency ultrasound, spina bifida screening and diabetes screening, determine if the fetus’ growth is on target (i.e. measuring too small or too big) and ultimately determine the timing of delivery should a complication arise or should the pregnancy extend beyond the due date.”
After eight weeks of pregnancy, the embryo is referred to as a fetus. At this stage, the fetus is developing a primary brain and brainstem, as well as early forms of the nose, mouth and eyes. Crucial muscles and bones are also beginning to develop. By the end of this trimester, the fetus is about three to four inches in length and weighs between half an ounce and one ounce. At this point, all major organs have formed. The fetus is able to move but often cannot be felt just yet. This stage is essential for early development and can provide a solid foundation for a healthy pregnancy. At the conclusion of the first trimester, the chance of miscarriage drops to 1% to 5%, according to March of Dimes.
The Second Trimester
The second trimester begins at 14 weeks and ends at 26 weeks. At 24 weeks, the rate of survival outside of the womb is between 60-70%, according to University of Utah Health.
Experts stress the importance of prenatal care prior to conception if possible. “A third of patients seek prenatal care after 13 weeks,” says Dr. Vernon. “Considering that the primary period of organ development is three to 13 weeks, intervening prior to conception to prevent congenital anomalies is more substantial than intervention at the first prenatal visit.”
During this period, your OBGYN tests for an antigen protein in your blood called Rh factor. If you’re negative for this protein, you’ll receive an injection later in pregnancy to protect the fetus. Without the injection, your body could create antibodies that could be harmful to the fetus during pregnancy and birth.
During the second trimester, you also receive an ultrasound to monitor fetal development and measure parts of the body. This imaging helps the doctor ensure everything is developing properly. Meanwhile, a glucose screening test measures how well your body is processing glucose in order to identify possible gestational diabetes. A sweet drink is provided for you, followed by a blood test an hour later. If the first test produces negative results, additional tests may be needed.
During the second trimester, the fetus develops all major organs and the umbilical cord continues to thicken. The placenta also fully develops, which provides oxygen and nutrients to the fetus. Movement may now be felt, and the fetus can now hear sounds from outside the body. The fetus also looks more like a baby, as the eyes are moving to the front of the head and the ears are moving to the sides. By the end of the second trimester, the fetus is typically between 13 and 16 inches long and weighs between two and three pounds.
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The Third Trimester
The third trimester begins at 27 weeks and lasts until the birth of the baby, around 40 weeks. At 28 weeks, the rate of survival outside the womb increases to 80% to 90%, according to University of Utah Health. Your OBGYN will likely schedule your appointments more frequently—every two weeks, then every week—to monitor you and the fetus in preparation for birth. You will likely have a final ultrasound, and your doctor may order a urine test to check for preeclampsia, a serious blood pressure condition. After 38 weeks, your OBGYN will perform an exam to check your cervix for signs of labor. You will also want to have a birth plan in place and discuss your wishes with your doctor.
Throughout this final stage, the fetus continues to gain weight and essential organs mature. Closer to labor, the head may move down into the pelvic region in preparation for birth. The fetus is now covered in vernix caseosa, a white protective film covering the skin. The skull bones continue to remain soft, which helps the head to pass through the pelvis and vaginal canal during delivery. By the end of the third trimester, the fetus is typically about 19 to 21 inches long and weighs between six and nine pounds.