Pregnancy Due Date Calculator

Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception. This is because the exact conception date is often unknown, while the LMP is easier to track. The standard medical formula, known as Naegele's rule, adds 280 days (40 weeks) to the LMP to estimate the expected date of delivery (EDD). This method was first described by the German obstetrician Franz Karl Naegele in the early 19th century and remains the clinical standard today.

Gestational age differs from fetal age by about two weeks. Gestational age counts from the LMP, whereas fetal age counts from conception. When you read that a fetus is 12 weeks old, doctors are referring to gestational age. The distinction matters for interpreting ultrasound reports and research studies.

Pregnancy Timeline

• Week 4-5: Positive pregnancy test; implantation occurs.
• Week 8-12: First ultrasound, heartbeat visible, organogenesis complete.
• Week 18-22: Anatomy scan, gender reveal possible, quickening felt.
• Week 24: Viability milestone; fetus may survive with intensive care.
• Week 37-42: Full term; baby can arrive any time.

For couples using assisted reproductive technology, IVF dating is more precise because the exact fertilization date is known. A day-3 embryo transfer adds 263 days to the transfer date, while a day-5 blastocyst adds 261 days. Preterm birth is defined as delivery before 37 weeks, and post-term pregnancy reaches 42 weeks or more.

You may also find our BMI Calculator, Calorie Calculator, and Sleep Calculator helpful during pregnancy planning.