Due Date Calculator
The Due Date Calculator estimates the delivery date of a pregnant woman based on her last menstrual period (LMP), ultrasound, conception date, or IVF transfer date.
Estimation of due date
The due date, sometimes called the estimated date of confinement, indicates when a pregnant woman is expected to give birth. Although many think of it as a single day, it's useful to view it as a window, because only about four percent of deliveries actually happen on that exact day.1
Due dates can be estimated using a number of different methods, including the last menstrual period, ultrasound, conception date, and IVF transfer date.
Last menstrual period
This tool calculates the expected delivery based on the woman's last menstrual period (LMP), assuming a typical pregnancy lasts 280 days, or 40 weeks, from that point. Whether pregnancy is said to begin at fertilisation or at implantation is a matter of discussion, but the gestational age we use always counts from the LMP. In practice, pregnancies range from 37 to 42 weeks, with 40 weeks serving as the standard reference, so the due date is obtained by adding 40 weeks to the first day of the LMP.
Ultrasound
When an ultrasound is used to predict the due date, sound waves create an image of the fetus, allowing its size to be compared with established growth charts worldwide. The procedure is quick, safe for the baby, and can give a reliable estimate early in the pregnancy.
Conception date
Estimating the delivery date from the conception date works much like using the last menstrual period, but the two methods differ by roughly two weeks, reflecting the interval between the start of the period and the moment of fertilisation.
In vitro fertilization (IVF)
In cases of in‑vitro fertilisation, the due‑date calculation can be more exact because the exact day of embryo transfer is known. While the standard 40‑week gestational benchmark from the LMP is still applied, IVF allows the estimate to be anchored to the LMP, ovulation day, egg retrieval, insemination, or the specific 3‑day or 5‑day embryo transfer date; this calculator relies on the embryo transfer date.
Due date as a reference point
The timing of birth within the 37‑to‑42‑week span generally isn’t worrisome. Babies delivered at 37‑39 weeks are classified as early‑term, those at 39‑41 weeks as full‑term, and 41‑42 weeks as late‑term; all three groups can be healthy, though full‑term infants tend to have the best outcomes.2 Births before 37 weeks are termed preterm, and those after 42 weeks are post‑term. These intervals help clinicians decide when intervention is needed. For instance, if labor begins at 33 weeks, doctors may try to halt it to prevent the complications of prematurity. Conversely, if a pregnancy extends beyond 42 weeks, induction may be advised because the placenta can begin to falter while the fetus still demands increasing nutrients and oxygen, eventually risking inadequate support.3
- Moore, Keith. 2015. "How accurate are 'due dates'?" BBC, February 3, 2015. bbc.com/news/magazine-31046144.
- Wikipedia: The Free Encyclopedia. 2017. en.wikipedia.org/wiki/Pregnancy.
- PubMed Health. 2014. "Pregnancy and birth: When your baby's due date has passed." ncbi.nlm.nih.gov/pubmedhealth/PMH0072755/.