Pregnancy Week by Week — Due Date, Trimesters and Milestones Explained
Finding out you’re pregnant changes everything — and suddenly every week feels important. But pregnancy can feel overwhelming when you’re trying to understand what’s happening inside your body, when your baby reaches major milestones, and how your due date is actually calculated.
This guide breaks it all down week by week, trimester by trimester. Whether you’re in your first few weeks or approaching your due date, you’ll find a clear picture of what to expect at every stage — and how to use our free Pregnancy Calculator to track it all in one place.
How Is a Pregnancy Due Date Calculated?
Your due date — also called the Estimated Due Date or EDD — is typically calculated as 280 days (40 weeks) from the first day of your last menstrual period. This is the standard method used by doctors worldwide, even though conception doesn’t actually happen on the first day of your period.
There are five methods to calculate your due date, and your doctor may use any of them depending on what information is available:
- Last Menstrual Period (LMP) — the most common starting point, assumes a 28-day cycle
- Conception Date — if you know when you conceived, add 266 days to get your due date
- Estimated Due Date — if your doctor has already given you a date, work backwards to find your LMP
- IVF Transfer Date — calculated using the embryo transfer date and embryo type (Day 3 or Day 5)
- Ultrasound — the most accurate method, especially in the first trimester when all embryos grow at the same rate
Keep in mind that only about 5% of babies are actually born on their exact due date. The due date is a target — most healthy pregnancies deliver anywhere between 37 and 42 weeks.
The Three Trimesters at a Glance
Pregnancy is divided into three trimesters, each lasting roughly 13 weeks. Here’s a quick overview of what happens in each stage:
| Trimester | Weeks | Key Changes | What to Expect |
| First Trimester | Weeks 1–13 | Heart forms, organs develop, morning sickness peaks | Fatigue, nausea, frequent urination, emotional changes |
| Second Trimester | Weeks 14–27 | Baby moves, gender visible, bump shows clearly | Energy returns, baby kicks felt, anatomy scan done |
| Third Trimester | Weeks 28–40 | Baby gains weight rapidly, lungs mature, brain develops | Back pain, Braxton Hicks, nesting instinct, preparing for birth |
First Trimester — Weeks 1 to 13
The first trimester is one of the most critical periods in your pregnancy, even though you may not look pregnant yet. This is when your baby goes from a single fertilized cell to a fully formed fetus with all major organs in place.
What’s Happening with Your Baby
By week 4, the embryo has implanted in the uterine wall and a pregnancy test will turn positive. By week 6, a heartbeat is usually detectable via ultrasound. By week 10, the embryo officially becomes a fetus — fingers, toes, and facial features are forming. By the end of week 13, all major organs are present and the miscarriage risk drops significantly.
What You Might Feel
- Morning sickness — nausea that can strike at any time of day
- Extreme fatigue — your body is working hard even if it doesn’t show
- Frequent urination as blood flow to your kidneys increases
- Breast tenderness and mood swings due to hormonal changes
- Heightened sense of smell and food aversions
This is also the trimester when your first prenatal appointment and early dating ultrasound typically happen. If you have irregular cycles, an ultrasound at 6 to 10 weeks gives the most accurate due date.
Second Trimester — Weeks 14 to 27
Most women feel significantly better in the second trimester. Morning sickness fades for many, energy returns, and your pregnancy becomes visible. This is often called the ‘golden trimester.’
What’s Happening with Your Baby
By week 16, your baby is the size of an avocado and can make facial expressions. Around week 18, movements become noticeable — that fluttery feeling is your baby kicking for the first time. By week 20, an anatomy scan checks all major organs and can usually reveal the baby’s sex. By week 23, a baby born prematurely has a chance of survival with medical support. By week 27, the baby can open and close their eyes.
What You Might Feel
- Visible baby bump appearing and growing week by week
- Baby kicks and movements — stronger as the weeks go on
- Back pain and round ligament pain as your uterus expands
- Increased appetite — your baby is growing fast
- Braxton Hicks contractions may start — practice contractions that aren’t real labor
Third Trimester — Weeks 28 to 40
The home stretch. Your baby is gaining weight rapidly and preparing for life outside the womb. This trimester can feel physically demanding, but every week brings you closer to meeting your baby.
What’s Happening with Your Baby
By week 28, the baby’s lungs are developing and they can now breathe rhythmically. Brain development accelerates rapidly. By week 32, the baby is practicing sucking and swallowing. By week 37, the baby is considered full term — their organs are mature and ready. Weeks 38 to 40 are the typical delivery window, with week 40 being the estimated due date.
What You Might Feel
- Shortness of breath as the baby pushes against your diaphragm
- Frequent urination returning as the baby drops lower
- Swelling in feet and ankles
- Stronger Braxton Hicks contractions
- Nesting instinct — a strong urge to prepare the home for baby
- Difficulty sleeping due to discomfort and frequent waking
Key Pregnancy Milestones Week by Week
Our free Pregnancy Calculator automatically highlights these milestones in your personalized week-by-week calendar:
| Week | Milestone |
| Week 4 | Pregnancy test turns positive — the embryo implants in the uterus |
| Week 6 | Heartbeat becomes detectable by ultrasound |
| Week 10 | All major organs are formed — fetus stage begins |
| Week 13 | Miscarriage risk drops significantly — first trimester ends |
| Week 18 | Baby starts making noticeable movements, can hear sounds, gender can be determined |
| Week 23 | A premature baby born at this point may survive with medical support |
| Week 28 | Baby can breathe — lungs are developing rapidly |
| Week 37 | Full term — baby is considered ready for birth |
| Week 40 | Estimated due date |
IVF Pregnancies — How Due Date Calculation Differs
If you conceived through IVF, your due date is calculated differently. Instead of counting from your last menstrual period, it’s based on your embryo transfer date and the age of the embryo:
- Day 3 embryo transfer — add 263 days to the transfer date for your due date
- Day 5 embryo transfer (blastocyst) — add 261 days to the transfer date
- Day 6 embryo transfer — add 260 days
Our calculator handles all three IVF embryo types automatically. Just select the IVF Transfer tab, enter your transfer date and embryo type, and it calculates your due date and full week-by-week calendar instantly.
Tips for Tracking Your Pregnancy Week by Week
- Start tracking early — the first trimester passes quickly and those early weeks matter
- Use your LMP date if you know it — it’s the standard starting point for all pregnancy dating
- Get an early ultrasound between 6 and 10 weeks for the most accurate due date
- Don’t fixate on the exact due date — think of it as the middle of a 5-week window (weeks 38 to 42)
- Track your trimester transitions — moving from first to second trimester at week 14 is a meaningful milestone
- Use our free calculator to see exactly how many weeks and days pregnant you are today
For detailed clinical information on pregnancy dating and gestational age, visit the For detailed clinical information on pregnancy dating, visit the American Pregnancy Association for trusted medical guidance.
Track Your Pregnancy with Our Free Calculator
Our free Pregnancy Calculator supports all five dating methods — LMP, conception date, due date, IVF transfer, and ultrasound. Once you enter your information, it instantly shows you:
- Your estimated due date
- Your current pregnancy status in weeks and days
- How many days until your due date
- Which trimester you’re in right now
- A complete week-by-week calendar from week 1 to week 42 with all major milestones highlighted
Try it now: Free Pregnancy Calculator — get your full week-by-week pregnancy timeline in seconds.
Frequently Asked Questions
| How many weeks is a full-term pregnancy? | A full-term pregnancy is 40 weeks, measured from the first day of your last menstrual period. Babies born between 37 and 42 weeks are considered full term. Babies born before 37 weeks are premature, and those born after 42 weeks are post-term. |
| What trimester am I in at 20 weeks? | At 20 weeks you are in the second trimester, which runs from week 14 to week 27. This is also the halfway point of a 40-week pregnancy. Around week 20 your doctor will typically schedule an anatomy scan to check fetal development. |
| Can my due date change during pregnancy? | Yes. If an early ultrasound shows a gestational age that differs by more than 7 days from your LMP-based due date, your doctor will usually adjust the due date to match the ultrasound. This is most accurate when done in the first trimester. |
| How is an IVF due date different from a natural pregnancy due date? | With IVF, the due date is calculated from the embryo transfer date rather than the last menstrual period. The calculation accounts for whether it was a Day 3 or Day 5 embryo transfer. This makes IVF dating more precise since the exact fertilization timeline is known. |
| What week do most women give birth? | Most babies are born between weeks 38 and 41. Only about 5% of births happen exactly on the due date. Week 40 is the estimated due date, but weeks 39 and 40 are statistically the most common delivery weeks for full-term pregnancies. |
Conclusion
Pregnancy is a 40-week journey with something new happening almost every single week. Understanding your trimesters, knowing when your baby hits key milestones, and tracking your due date accurately takes the guesswork out of what can feel like an overwhelming experience.
Whether you’re calculating from your last period, a conception date, an IVF transfer, or an ultrasound, the most important thing is to start tracking early and stay consistent with your prenatal care.
Use our free Pregnancy Calculator to get your personalized week-by-week pregnancy calendar — and see exactly where you are in your journey today.