Figuring out how many weeks pregnant you are starts with identifying the first day of your last menstrual period and counting forward to today. Pregnancy lasts approximately 40 weeks (280 days), and your estimated due date falls at the end of that timeline. An OBGYN can confirm your gestational age through ultrasound imaging and clinical evaluation, ensuring you have accurate dates for planning your prenatal care.
How to Calculate Your Due Date Step by Step
The standard method for calculating your due date uses a formula called Naegele’s Rule. This approach provides a quick estimate that your OBGYN will verify at your first prenatal appointment.
The Naegele’s Rule Calculation
To calculate your due date using Naegele’s Rule, take the first day of your last menstrual period, add one year, subtract three months, and add seven days. For example, if your last period started on March 10, 2025, your estimated due date would be December 17, 2025.
Here is the formula broken down:
- Step 1: Note the first day of your last menstrual period (LMP)
- Step 2: Add one year to that date
- Step 3: Subtract three months
- Step 4: Add seven days
This calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycles run longer or shorter than 28 days, your OBGYN specialists may adjust your due date accordingly during your first prenatal visit.
Counting Your Pregnancy Weeks
Once you know your LMP date, counting your pregnancy weeks becomes straightforward. Week one begins on the first day of your last menstrual period, even though conception typically occurs around week two or three. This means you are technically “pregnant” for about two weeks before the egg and sperm actually meet.
To determine how many weeks pregnant you are today, count the number of complete weeks between your LMP date and the current date. If your last period started eight weeks and four days ago, you are eight weeks and four days pregnant. Your OBGYN clinic uses the same counting method and expresses gestational age in weeks and days (e.g., 8+4 or 8w4d in medical records).
Sample Due Date Calculations
The table below shows sample LMP dates and their corresponding due dates and current gestational ages:
| Last Menstrual Period | Estimated Due Date | Weeks Pregnant (as of Jan 20, 2026) |
| April 14, 2025 | January 19, 2026 | 40 weeks, 1 day |
| May 12, 2025 | February 16, 2026 | 36 weeks, 1 day |
| June 9, 2025 | March 16, 2026 | 32 weeks, 1 day |
| July 7, 2025 | April 13, 2026 | 28 weeks, 1 day |
| August 4, 2025 | May 11, 2026 | 24 weeks, 1 day |
| September 1, 2025 | June 8, 2026 | 20 weeks, 1 day |
Your OBGYN uses these calculations as a starting point and may adjust your due date based on ultrasound findings, particularly if measurements differ by more than seven days in the first trimester.
When Ultrasound Dating Provides Better Accuracy
While LMP-based calculations work well for many women, ultrasound measurements offer greater precision in certain situations. Your OBGYN care team determines which dating method applies best to your pregnancy.
First-Trimester Ultrasound Measurements
Ultrasounds performed between 6 and 13 weeks provide the most accurate pregnancy dating, with precision within five to seven days. During this window, your OBGYN measures the crown-rump length (CRL), which is the distance along your baby’s body. Early embryonic and fetal development follows highly predictable patterns, making these measurements reliable indicators of gestational age.
If your first-trimester ultrasound suggests a due date that differs by more than seven days compared to your LMP calculation, your provider will likely adjust your official due date to match the ultrasound findings. This adjustment ensures accurate timing for prenatal screenings and delivery planning.
Situations Requiring Ultrasound Dating
Several circumstances make ultrasound dating necessary rather than optional. Women who cannot remember their last menstrual period date need ultrasound confirmation of gestational age. Those with irregular menstrual cycles (cycles shorter than 24 days or longer than 35 days) also benefit substantially from ultrasound dating, since LMP calculations assume a regular 28-day cycle.
Women who conceive while breastfeeding, those who had bleeding early in pregnancy that could be confused with a period, and patients who recently stopped hormonal contraception may have unreliable LMP dates. Your OBGYN specialists identify these situations and schedule early ultrasounds to establish accurate pregnancy dating. IVF patients have exact conception dates and use transfer-date calculations, which your prenatal care team will incorporate into your records.
Understanding Your Pregnancy Timeline by Trimester
Knowing how many weeks pregnant you are helps you track which trimester you have entered and what developmental milestones to expect. Each trimester spans roughly 13 weeks and brings distinct changes.
First Trimester Milestones (Weeks 1 Through 13)
The first trimester covers weeks 1 through 13 of pregnancy. During this time, your baby develops rapidly, forming all major organ systems by week 12. You will likely experience symptoms such as nausea, fatigue, breast tenderness, and frequent urination as your body adjusts to pregnancy hormones.
Your OBGYN clinic schedules your first prenatal appointment during this trimester to confirm your pregnancy, establish dating, and order initial blood work. Early prenatal vitamin supplementation supports healthy neural tube development. Genetic screening options become available between weeks 10 and 13.
Second Trimester Milestones (Weeks 14 Through 27)
The second trimester spans weeks 14 through 27. Many women find this trimester more comfortable as early pregnancy symptoms subside and energy levels improve. You will likely feel your baby’s first movements (called quickening) between weeks 18 and 22.
Key appointments during this trimester include the anatomy ultrasound around week 20 and glucose screening between weeks 24 and 28. Your OBGYN care team monitors fetal growth, evaluates organ development, and checks placental position during these visits.
Third Trimester Milestones (Weeks 28 Through 40)
The third trimester begins at week 28 and continues until delivery, typically around week 40. Your baby gains weight rapidly during this period, develops lung maturity, and moves into a head-down position in preparation for birth. You may experience increased discomfort, including back pain, heartburn, and difficulty sleeping.
Prenatal visits become more frequent during the third trimester, occurring every two weeks and then weekly as your due date approaches. Your OBGYN monitors fetal heart rate, your blood pressure, and signs of labor during these appointments. Women with high-risk pregnancies may have additional monitoring and testing throughout this trimester.
How Accurate Is Your Due Date?
Your estimated due date represents your best prediction for delivery, but most babies arrive within a two-week window surrounding that date. Understanding due date accuracy helps you prepare for the range of possible delivery timings.
What the Research Shows
Studies indicate that only about 5% of babies arrive on their exact due date. The majority of deliveries occur between 37 and 42 weeks of pregnancy. First-time mothers tend to deliver slightly later than their due dates, while women who have given birth before may deliver somewhat earlier.
Full-term pregnancy is defined as delivery between 39 weeks and 40 weeks, 6 days. Babies born during this window generally have optimal outcomes, including better brain development and a lower risk of respiratory complications. Your OBGYN specialists discuss the benefits of allowing pregnancy to reach full term when medically appropriate.
Factors That Influence Delivery Timing
Several variables affect when you actually deliver compared to your calculated due date:
- Accuracy of dating: Earlier ultrasounds provide more reliable dates than later ones
- Menstrual cycle length: Longer cycles may mean later ovulation and later delivery
- First versus subsequent pregnancies: First babies often arrive later
- Multiple gestations: Twins and triplets typically deliver earlier
- Medical conditions: Certain health factors may necessitate earlier delivery
Your OBGYN care team takes these factors into account when discussing your expected delivery timeline and preparing you for labor and birth.
Schedule Your Prenatal Appointment at Roswell Ob/Gyn, LLC
Confirming your due date and gestational age early in pregnancy sets the foundation for excellent OBGYN care throughout your journey. Your first prenatal visit allows your provider to verify your dates, order appropriate screenings, and answer your questions about what lies ahead.
Roswell Ob/Gyn, LLC offers comprehensive prenatal services at our OBGYN clinic locations in Alpharetta, Atlanta, Canton, and Cumming, GA. Our OBGYN specialists provide in-office ultrasound imaging, personalized pregnancy guidance, and expert care for both routine and high-risk pregnancies. Schedule your appointment today to confirm how many weeks pregnant you are and begin your prenatal care with confidence.
