Pregnancy Calculator Guide — Due Dates, Trimesters & Baby Development
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.
Pregnancy is measured 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
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
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
Your baby goes from a tiny ball of cells to a fully formed little fetus with a beating heart, arms, legs and organs. Many people feel tired, bloated and nauseous in this phase.
- 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.
Second Trimester — Weeks 14 to 27
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.
- 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
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.
- 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.
How Accurate Is Your Due Date?
It's completely normal if your baby doesn't arrive exactly on the due date. 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.
Useful Tools to Combine with This Calculator
- 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.
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.