Pregnancy Calculator Guide 2025 – Due Dates, Trimesters & Baby Development (US, UK & Worldwide)
Seeing a positive test is exciting, but the first question most people ask is: “When is my baby due?” Our pregnancy calculator is built to give you the same style of dates your doctor or midwife will use – just explained in simple, friendly language.
Whether you live in the United States, United Kingdom, Canada, Australia, Europe or anywhere else, pregnancy is measured the same way: from the first day of your last menstrual period (LMP). A full-term pregnancy is about 40 weeks (280 days) from LMP or 38 weeks from conception.
How our pregnancy calculator works (step by step)
This calculator uses the same medical rules used in clinics and hospitals:
- LMP method: add 280 days (40 weeks) to the first day of your last period.
- Conception method: add 266 days (38 weeks) to the day you conceived or ovulated.
- Ultrasound method: work backwards from the ultrasound date and the gestational age measured on the scan.
You can switch between methods easily. This is helpful if:
- your cycles are longer or shorter than 28 days,
- you conceived via IVF and know the transfer date,
- or your doctor changed your due date after an early scan.
The three trimesters: what’s happening to you and your baby
Doctors divide pregnancy into three main stages. Our tool shows the exact calendar dates for each trimester based on your data:
First trimester – weeks 1 to 13
The first trimester is all about foundations. Your baby goes from a tiny ball of cells to a fully formed little fetus with a beating heart, arms, legs and organs. Many women in the US, UK and elsewhere feel tired, bloated and nauseous in this phase.
Typical first-trimester highlights:
- Heart starts beating (around weeks 6–7, seen on ultrasound).
- Face, limbs and tiny fingers begin to form.
- Miscarriage risk drops sharply after week 12.
This is also when you’ll have your first prenatal visit and blood tests, and often a dating scan. Our calculator plots these key weeks so they’re easy to see at a glance.
Second trimester – weeks 14 to 27
Many parents call this the “sweet spot.” Energy usually returns, your bump starts to show, and you feel those first kicks. By now your baby has fingerprints, can hear your voice and has regular sleep-wake cycles.
Common second-trimester milestones include:
- Feeling baby move (often between 16–22 weeks).
- The big anatomy scan around 20 weeks.
- Glucose screening for gestational diabetes (24–28 weeks).
This is also a great time to use the Budget Calculator and Percentage Calculator to plan maternity leave, baby shopping and savings.
Third trimester – weeks 28 to birth
In the third trimester your baby packs on weight, the lungs mature, and the brain grows at high speed. You may feel more breathless, tired and uncomfortable as your due date gets closer – that’s completely normal.
Key third-trimester checkpoints:
- Start of weekly or bi-weekly prenatal appointments.
- Group B strep test (often 35–37 weeks).
- Baby usually moves into head-down position.
- “Early term” at 37 weeks, “full term” at 39–40 weeks.
Our pregnancy calculator shows all of this on a simple timeline, including when you’re likely to hit full term and when most doctors in the US, UK and other countries start extra monitoring if you go past your due date.
How accurate is your due date really?
It’s completely normal if your baby doesn’t arrive exactly on the due date. In fact, only about 1 in 20 babies are born on that exact day. Most arrive sometime between 38 and 41 weeks.
- LMP: usually accurate if your cycles are regular.
- Early ultrasound: often accurate within 3–5 days.
- Later ultrasounds: less precise because babies grow at different speeds.
Think of the due date as the center of a window, not a deadline. Your doctor or midwife will guide you if induction is recommended after 41–42 weeks, depending on local guidelines in your country.
Useful tools to combine with the pregnancy calculator
To get a complete picture of your health during pregnancy, you can use this tool alongside these calculators:
- BMI Calculator – track pre-pregnancy and early pregnancy BMI.
- BMR Calculator – understand your baseline energy needs.
- Calorie Calculator – talk with your provider about healthy weight gain.
- Date Calculator – work out appointment dates, maternity leave and travel plans.
- Age Calculator – check maternal age at due date for risk discussions.
None of these tools replace your obstetrician, midwife or family doctor – but they make it much easier to walk into your appointments already informed and prepared.