Pregnancy Calculator Guide: Due Date, Trimesters & Milestones
Calculate your due date, understand the three trimesters, and know exactly what to expect week by week during your pregnancy journey.
How Due Date Is Calculated
The standard method (Naegele's Rule) calculates due date from the first day of your last menstrual period (LMP):
Due Date = LMP + 280 days (40 weeks)
Or: LMP + 9 months + 7 days
This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, our pregnancy calculator adjusts accordingly.
A first-trimester ultrasound (crown-rump length measurement before 14 weeks) is the most accurate dating method and may revise your due date.
Trimester Breakdown
Pregnancy is divided into three trimesters, each with distinct developmental phases and physical changes for the mother.
First Trimester: Weeks 1–13
Organ formation, morning nausea, extreme fatigue. Miscarriage risk is highest in this period — most women wait to announce until after week 12.
Second Trimester: Weeks 14–27
Often called the "honeymoon trimester." Nausea fades, energy returns. Baby movements (quickening) felt at ~18–22 weeks. Anatomy scan at ~20 weeks.
Third Trimester: Weeks 28–40+
Baby gains 50% of birth weight. Braxton Hicks contractions increase. Cervix begins effacing and dilating near term.
Key Pregnancy Milestones
Track the most important events across all 40 weeks of pregnancy:
| Week | Milestone |
|---|---|
| Week 4 | Implantation complete; home test positive |
| Week 6–8 | Heartbeat detectable by ultrasound |
| Week 10 | All major organs formed |
| Week 12 | End of 1st trimester; miscarriage risk drops significantly |
| Week 16–20 | First movements felt (quickening) |
| Week 20 | Anatomy scan ultrasound |
| Week 24 | Viability threshold (survival possible with intensive care) |
| Week 28 | Third trimester begins; GBS testing prep |
| Week 32–36 | Baby drops, lung maturation completes |
| Week 39–40 | Full term; optimal delivery window |
Prenatal Care Schedule
A consistent prenatal care schedule ensures both mother and baby are monitored at each critical stage of development.
FAQ
How accurate is a due date calculated from LMP?
What percentage of babies are born on their due date?
When does morning sickness typically end?
When will I start showing?
What week does the third trimester start?
Author Spotlight
The ToolsACE Team
ToolsACE is an independent platform founded in 2023 by a team of software developers and educators. We build free, privacy-first tools and write guides to help people make better decisions — without sign-ups, paywalls, or data tracking.
