Pregnancy Calculator

Pregnancy term and the due date

Pregnancy — the roughly nine months during which a baby develops — is dated from the first day of the last menstrual period, an observable date, rather than from conception, which usually is not. Birth typically follows about 38 weeks after conception, which is 40 weeks after the last period, and the World Health Organization puts a normal term between 37 and 42 weeks. At the first obstetric visit, a due date is usually estimated from a sonogram; it can also be estimated from the last period, as this calculator does. Treat the date as the center of a window: the actual length of pregnancy varies with the mother's age, previous pregnancy lengths, and her own birth weight, among factors still not fully understood — fewer than 4% of births land on the exact due date, about 60% land within a week of it, and almost 90% within two weeks.

Detecting pregnancy

Pregnancy announces itself through signs such as a missed period, raised basal body temperature, fatigue, nausea, and more frequent urination — and is confirmed by tests that detect hCG, a hormone present only in pregnancy. Clinical blood tests can detect it six to eight days after fertilization, measure exact amounts, and catch a pregnancy earlier and at lower hormone levels than urine tests, at the cost of time and money. Home urine tests are cheaper and fast; clinical urine tests are not necessarily more accurate than a good home test.

Managing a pregnancy

Medication. Some medicines can have lasting effects on the fetus, so every medication planned during pregnancy should be cleared with a doctor, who weighs the benefit to the mother against the risk to the baby. Weight gain is necessary and expected; the Institute of Medicine recommends overall gains of 28–40 lb for women underweight before pregnancy (BMI under 18.5), 25–35 lb for normal weight (BMI 18.5–24.9), 15–25 lb for overweight (BMI 25–29.9), and 11–20 lb for obesity (BMI over 30) — our Pregnancy Weight Gain Calculator is built on these guidelines. Too little or too much gain raises risks, including gestational hypertension and cesarean delivery. Exercise: aerobic exercise helps maintain fitness and may reduce the chance of a C-section; women who trained regularly before pregnancy and have uncomplicated pregnancies can often continue, but the American College of Obstetricians and Gynecologists advises stopping and consulting a doctor if symptoms such as vaginal bleeding, dizziness, headache, chest pain, calf pain or swelling, leaking fluid, decreased fetal movement, or preterm labor appear. Nutrition: pregnancy raises energy and micronutrient needs — folic acid (vitamin B9) lowers the risk of certain birth defects, and DHA omega-3, needed for brain and retinal development, reaches the baby through the placenta. A doctor or dietitian can tailor the details to your situation.

References

1. Jukic AM, Baird DD, Weinberg CR, et al. "Length of human pregnancy and contributors to its natural variation." Human Reproduction 28(10), 2013.
2. Moore, Keith. "How accurate are 'due dates'?" BBC, February 3, 2015.
3. Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press, 2009.
4. Davies GA, Wolfe LA, Mottola MF, et al. "Exercise in pregnancy and the postpartum period." Journal of Obstetrics and Gynaecology Canada 25(6), 2003.
5. Artal R, O'Toole M. "Guidelines of the ACOG for exercise during pregnancy and the postpartum period." British Journal of Sports Medicine 37, 2003.
6. Lammi-Keefe CJ, Couch SC, Philipson E. Handbook of Nutrition and Pregnancy. Humana Press, 2008.