How to Calculate Your Pregnancy Due Date (And What to Wear Each Trimester)

Finding out you're pregnant is one of life’s most exciting moments — and one of the first questions every expecting mom asks is: when is my baby due? Knowing your due date helps you plan prenatal appointments, prepare your nursery, schedule your baby shower, and — yes — build the perfect maternity wardrobe for each stage of your pregnancy.

In this guide, we’ll walk you through exactly how due dates are calculated, how to use our free pregnancy due date calculator, and what to wear during each trimester to look and feel your best.

How Is a Pregnancy Due Date Calculated?

Most due dates are estimated using Naegele’s Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your ovulation window shifts — and so does your estimated due date.

There are actually four common methods used to estimate your due date:

• Last Menstrual Period (LMP) — The most common method. Enter the first day of your last period, and the calculator adds 280 days.
• Conception Date — If you know exactly when you ovulated or conceived, your due date is approximately 266 days (38 weeks) from that date.
• IVF Transfer Date — For IVF pregnancies, due dates are calculated based on the embryo’s age at transfer (Day 3 or Day 5 blastocyst).
• Ultrasound Dating — The most accurate method, especially in the first trimester. Your provider measures the baby and assigns a gestational age.

Not sure which method applies to you? Our free pregnancy due date calculator supports all four methods and gives you your EDD, current trimester, and key milestones in seconds.

Real Stories: Three Ways Moms Calculate Their Due Dates

Sarah — The First-Time Mom with Regular Cycles

Sarah got a positive pregnancy test and immediately wanted to know her due date. Her last period started on March 1st, and she has a regular 28-day cycle. Using the LMP method on our due date calculator, she discovered her EDD is December 6th — putting her right in the middle of her second trimester when her baby shower rolls around in October. She’s already eyeing our Off Shoulder Lace Maternity Photoshoot Dress for her celebration.

Jessica — The Mom with an Irregular Cycle

Jessica has always had irregular periods, ranging from 24 to 35 days. The standard LMP method wasn’t reliable for her. Instead, she tracked ovulation with an app and knew her conception date. She entered it into the Conception Date tab of our calculator and got a much more accurate estimate. Her OB later confirmed the date with an early ultrasound — it was off by just two days. For her growing bump in the second trimester, she loves the effortless silhouette of our Solid Chiffon Sleeveless Maternity Maxi Dress.

Mia — The IVF Mom

Mia went through IVF and had a Day 5 blastocyst transfer on February 15th. Traditional due date methods didn’t apply to her, but the IVF tab on our free pregnancy due date calculator handled it perfectly — automatically calculating her due date as October 29th. Now at 30 weeks, she’s planning her maternity photoshoot and has her eye on the stunning Floral Print Maternity Long Photoshoot Dress that photographs beautifully.

What to Wear Each Trimester

Your body changes significantly across all 40 weeks of pregnancy — and your wardrobe should keep up. Here’s a trimester-by-trimester style guide to keep you comfortable, confident, and gorgeous.

First Trimester (Weeks 1–13): Comfort Is Queen

During the first trimester, your bump may not be visible yet — but bloating, fatigue, and morning sickness are very real. Focus on soft, stretchy fabrics that won’t add pressure to your midsection. Maternity leggings, flowy tops, belly bands, and wrap blouses are your best friends right now. You don’t need a full maternity wardrobe yet — a few key pieces will carry you through.

Second Trimester (Weeks 14–27): Hello, Bump!

This is the trimester most moms call the “honeymoon phase.” Your bump is beautifully visible, your energy is back, and you’re glowing. This is the time to invest in proper maternity dresses, jeans, and occasion wear. If you have a baby shower or gender reveal planned, the second trimester is when you’ll want to look your absolute best.

The Plus Size Solid Soft Short Baby Shower Maternity Dress is a perfect second-trimester pick — soft, stretchy, and flattering for every body type. For a more formal event, our Strapless Open-back Maternity Photoshoot Dress turns heads at any occasion.

Third Trimester (Weeks 28–40+): Bump in All Its Glory

By the third trimester, comfort and style go hand in hand. Maxi dresses and flowy gowns are ideal — they accommodate your growing bump without any tightness around the belly. This is also the prime time to book your maternity photoshoot. A professional session during weeks 30–36 captures your bump at its most beautiful.

For your photoshoot, our Off Shoulder Plus Size Maternity Photoshoot Long Dress is a bestseller — its off-the-shoulder silhouette and flowing length photograph magnificently. Pair it with soft lighting and an outdoor setting for timeless maternity portraits.

When to Book Your Maternity Photoshoot

Most photographers recommend booking your maternity session between weeks 28 and 36. This window gives you a beautifully round bump while still leaving you comfortable enough to pose for an extended session. After week 36, fatigue and discomfort often make photography sessions more challenging.

Once you know your due date — which you can find instantly using our free pregnancy due date calculator — count back from your EDD to schedule your session at the perfect time.

Frequently Asked Questions

How accurate is an online due date calculator?

Online due date calculators provide a reliable estimate, but only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the EDD. For the most accurate dating, an early first-trimester ultrasound (between 7 and 12 weeks) is the gold standard.

What if my periods are irregular?

If your cycle is irregular, the LMP method may not give an accurate estimate. Use the Conception Date method if you tracked ovulation, or rely on your first ultrasound. Always tell your OB or midwife about irregular cycles so they can order early dating imaging.

Can my due date change after an ultrasound?

Yes — this is called “redating.” If your ultrasound measurements differ significantly from your LMP-based date (usually more than 5–7 days in the first trimester), your provider may adjust your official due date. This is completely normal and actually makes your EDD more accurate.

What size maternity clothes should I buy for each trimester?

Generally, your pre-pregnancy size works as a starting guide. In the first trimester, stretchy styles in your regular size are usually fine. By the second trimester, invest in proper maternity sizing — designed with extra room in the bump area. For the third trimester, choose styles with maximum stretch and adjustable fits. When in doubt, size up — you can always tie or belt a looser dress.

When should I start wearing maternity clothes?

There’s no set rule — wear maternity clothes whenever you feel more comfortable in them. Most moms make the switch sometime between 12 and 16 weeks, when regular waistbands start feeling too tight. Some switch even earlier due to bloating in the first trimester. Listen to your body!

Ready to Find Your Due Date?

Your pregnancy journey is unique — and so is your due date. Whether you’re calculating by last period, conception date, IVF transfer, or ultrasound, our tool gives you your EDD, trimester, key milestones, and even personalized style recommendations for your current stage.

👉 Calculate your due date now — it’s free!

And when you’re ready to dress that beautiful bump, explore our full collections of maternity photoshoot gowns and baby shower dresses — designed to make every milestone moment unforgettable.