How is the due date calculated? Medically, the due date is calculated from the first day of the last menstrual period, with the entire pregnancy lasting 280 days or 10 gestational months (each month being 28 days). Deliveries occurring between 38 and 42 weeks of pregnancy are considered full-term.Since menstrual cycle lengths vary among women, a 1–2 week discrepancy between the estimated and actual due date is normal. The due date serves primarily as a reference; only 53% of babies are born on this day.
1. Last Menstrual Period (LMP) Method: Add 9 months or subtract 3 months from the month of your last period, then add 7 days to the day (referring to the first day of your last period).Example: If the last menstrual period began on March 5th and ended on March 10th, the estimated due date would be: Month = 3 + 9 = December Date = 5 + 7 = 12 Therefore, the due date is December 12th. Applicable to: This method is suitable for expectant mothers who can clearly recall their last menstrual period and have a regular menstrual cycle (approximately 28 to 32 days).In fact, many online due date calculators and hospital-provided due date calculators use this method.
2. Ultrasound Measurement Method: By measuring the baby's physical data during an ultrasound—such as biparietal diameter (BPD), head circumference, femur length, abdominal circumference, etc. (similar to an adult's three measurements)—the due date can also be estimated.
Below are relevant ultrasound measurements.Don't worry if you don't understand these measurements. Your doctor will calculate the gestational age (the baby's age in weeks; typically, 40 weeks of gestation corresponds to the due date) based on these data, so you won't need to do the math yourself. Once calculated, the doctor will note it on the ultrasound report as: Gestational Age ** weeks + * days.Typically, the gestational sac becomes visible in the uterine cavity via ultrasound around 35 days after the last menstrual period. At 6 weeks gestation, the sac's diameter is approximately 2 cm, increasing to about 5 cm by 10 weeks. FE — Embryo: Before the baby takes shape, it resembles a tiny bean sprout. The embryo becomes detectable on ultrasound between 6-7 weeks gestation.CRL — Crown-Rump Length: The distance from the top of the baby's head to the bottom of the buttocks, primarily used to determine gestational age between 7 and 12 weeks. To estimate gestational age (weeks) between 6 and 13 weeks: Gestational Age (weeks) = Crown-Rump Length + 6.5 cm.At full term, it should reach 9.3 cm or more. Generally, after the fifth month of pregnancy, it aligns with the gestational month. For example, at 28 weeks (7 months), BPD is approximately 7.0 cm, and at 32 weeks (8 months), it is about 8.0 cm, and so on. After the eighth month, an average weekly increase of about 0.2 cm is considered normal.(For mothers who cannot recall their menstrual cycle start date or have irregular cycles, ultrasound measurements can estimate fetal size to calculate the due date.)
How is the due date calculated? Medically, the due date is calculated from the first day of the last menstrual period, with the entire pregnancy lasting 280 days, or 10 gestational months (each month being 28 days).Delivery occurring between 38 and 42 weeks of gestation is considered full-term.
FL measurement is typically performed around 20 weeks gestation to assess fetal development. If the ultrasound-calculated due date differs from the menstrual-based date—showing the fetal age being one or two weeks larger or smaller—there's no need for excessive concern. Some measurements may gradually adjust. If smaller, eat more to help the baby catch up; if larger, manage your diet and avoid overeating.As long as the menstrual cycle calculation holds true, by week 37, everyone should be ready to welcome the baby at any moment. Therefore, there's no need to dwell on discrepancies between the ultrasound due date and the actual due date calculated from the menstrual cycle.Other methods for calculating due dates: Beyond the two convenient and accurate methods above, there are also the morning sickness method (typically occurring after 6 weeks, though individual variations exist, making it less reliable) and the first fetal movement method (usually felt between 16-18 weeks, but this relies on the movement being detected and isn't highly accurate, so it's not recommended).