Pregnancy Due Date Calculator

How Does the Formula Work?

The pregnancy due date calculator estimates your expected delivery date using Naegele's Rule — the standard method used by obstetricians worldwide since the 19th century. Named after German obstetrician Franz Karl Naegele, the rule calculates the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). If you know your conception date instead, the calculator adds 266 days (38 weeks) since fertilization typically occurs about 14 days after the LMP. Only about 5 percent of babies are born on their exact due date — the EDD represents the midpoint of a normal delivery window spanning 37 to 42 weeks of gestation.

How Naegele's Rule Works

The calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. For women with longer or shorter cycles, the actual ovulation date may differ, which is why first-trimester ultrasound dating (at 8 to 12 weeks) is more accurate for confirming the due date. If the ultrasound-derived date differs from the LMP-derived date by more than 7 days, most practitioners adopt the ultrasound date. The American College of Obstetricians and Gynecologists (ACOG) recommends using the best estimate from early ultrasound when available. This calculator provides the initial LMP-based estimate — discuss any adjustments with your healthcare provider.

Trimester Milestones

The first trimester (weeks 1 through 12) is the period of rapid embryonic development. By week 6 the heart begins beating. By week 8 all major organs have started forming. The first prenatal visit typically occurs at 8 to 10 weeks and includes blood tests, urine analysis, and often the first ultrasound. Morning sickness peaks between weeks 8 and 12. The risk of miscarriage drops significantly after the first trimester — from about 15 percent in the first 12 weeks to less than 1 percent afterward. The NIPT (non-invasive prenatal testing) blood test for chromosomal conditions like Down syndrome is available from week 10. In the United States, prenatal care is typically provided by an OB-GYN or certified nurse-midwife, with visits every 4 weeks during the first and second trimesters.

The second trimester (weeks 13 through 27) is often called the most comfortable period. The anatomy scan ultrasound at 18 to 22 weeks examines fetal organs, growth, and can reveal the sex. Fetal movement (quickening) is first felt between weeks 16 and 22. The glucose tolerance test for gestational diabetes is performed at 24 to 28 weeks. Weight gain of 1 to 2 pounds per week is normal during this period. Prenatal vitamins with folic acid, iron, and DHA should continue throughout pregnancy — brands like Nature Made, One A Day, and Vitafusion are widely available at CVS, Walgreens, and Target.

The third trimester (weeks 28 through 40) brings rapid fetal growth. By week 28 the baby weighs about 2.5 pounds. Group B Streptococcus (GBS) screening occurs at 36 weeks. Visits increase to every 2 weeks from 28 to 36 weeks, then weekly until delivery. The baby is considered full term at 39 weeks. Induction of labor may be recommended after 41 weeks if labor hasn't begun naturally. Birth plans, hospital bag preparation, and childbirth education classes (Lamaze, Bradley Method, hypnobirthing) typically happen during this trimester. Hospital delivery costs in the US average $13,000 for vaginal delivery and $22,000 for C-section before insurance — most insurance plans cover maternity care with copays and deductibles.

Prenatal Care Schedule in the US

ACOG recommends the following visit schedule for uncomplicated pregnancies: every 4 weeks through week 28, every 2 weeks from weeks 28 to 36, then weekly from week 36 until delivery — approximately 12 to 14 visits total. Each visit includes blood pressure measurement, weight check, urine protein/glucose screening, fundal height measurement, and fetal heart rate monitoring. Major tests include first-trimester screening (weeks 11–13), anatomy scan (weeks 18–22), glucose tolerance test (weeks 24–28), and GBS screening (week 36). Prenatal care is covered by all ACA marketplace plans and Medicaid in every state.

Nutrition and Exercise During Pregnancy

Caloric needs increase by 300–400 calories daily from the second trimester. Folic acid (400 micrograms daily) is critical before and during the first trimester to prevent neural tube defects — the CDC recommends all women of childbearing age take folic acid daily. Iron supplementation prevents gestational anemia, especially in the second and third trimesters. Omega-3 DHA supports fetal brain development — 200 milligrams daily from fish oil or algae supplements is recommended by ACOG. Moderate exercise — walking, swimming, prenatal yoga, and stationary cycling — is safe and recommended at 150 minutes per week. Apps like Ovia Pregnancy, The Bump, and What to Expect are popular in the US for tracking weekly development milestones and managing prenatal appointments.

Remember that the estimated due date is just a guideline — your healthcare provider will adjust it based on ultrasound measurements and your individual health profile throughout pregnancy.