Due Date Calculator — Calculate Your Baby's Due Date by LMP or Conception

A due date calculator estimates your baby's expected birth date using one of four methods: last menstrual period (LMP), known conception date, IVF day-3 embryo transfer, or IVF day-5 blastocyst transfer. It applies the standard obstetric formula — Naegele's rule for natural conception, or embryo-age-adjusted formulas for IVF — to produce the estimated due date, current gestational age, trimester, days remaining, and a timeline of key pregnancy milestones.

The estimated due date (EDD) is one of the first and most important pieces of information established in prenatal care. It defines the timeline for all subsequent care: when screenings should be performed, when the fetus reaches viability, when to expect delivery, and when interventions may be appropriate. While only about 4% of births occur on the exact EDD, the 40-week framework around which the EDD is centered guides the structure of the entire pregnancy.

Understanding gestational age — the standard clinical measure counted from LMP — is essential for interpreting all pregnancy information, from apps and books to medical reports. 'You are 8 weeks pregnant' means 8 weeks from LMP, which is only about 6 weeks of actual fetal development. This consistent offset between gestational and fetal age affects how parents interpret size charts, developmental milestones, and the meaning of prenatal screening results.

Example 1 — LMP method: LMP: January 10, 2025 Due date: October 17, 2025 (280 days later) Estimated conception: January 24, 2025 At week 20 (May 23): Anatomy scan due. Second trimester. At week 37 (September 19): Full-term threshold reached.

Example 2 — IVF day-5 blastocyst transfer: Transfer date: February 3, 2025 Due date: February 3 + 261 days = October 22, 2025 LMP equivalent: February 3 − 19 days = January 15, 2025 Gestational age at transfer: 5 weeks exactly (5+0).

Example 3 — Cycle length adjustment: LMP: January 1, 2025. Cycle length: 35 days (not 28). Standard Naegele due date: October 8, 2025 Adjusted due date: October 8 + 7 days = October 15, 2025 Ultrasound at 12 weeks will confirm actual dating.

The estimated due date is more than a calendar target — it is the reference point for the entire structure of prenatal care. Every screening, test, and developmental milestone in pregnancy is scheduled relative to gestational age, which is anchored to the EDD. The first-trimester nuchal translucency screening must be performed between weeks 11 and 14 — outside this window, the measurement is not accurate. The anatomy scan at week 20 is timed when fetal anatomy is large enough to assess in detail. Group B Streptococcus (GBS) testing occurs at weeks 36–37. Understanding your EDD precisely ensures you receive these screenings within their optimal windows.

For many families, the EDD is also the first organizing date of early parenthood — maternity and paternity leave requests, childcare waitlists (which often have 6–12 month waiting periods), hospital pre-registration, and childbirth education classes are all coordinated around it. Planning for a birth 3 weeks before or after the EDD is reasonable, given that only 4% of babies arrive on the exact date. Many experienced parents and midwives describe the due date as the center of a 'due month' rather than a precise target.

In the context of IVF and assisted reproduction, precise dating is particularly important because IVF pregnancies involve exact knowledge of embryo age that natural conception cannot provide. IVF-specific calculations have been validated in large clinical studies and are consistently more accurate than LMP-based dating for IVF pregnancies. Combining IVF transfer date with the appropriate embryo age adjustment (day-3 or day-5) gives the most precise gestational age calculation available — often more accurate than even first-trimester ultrasound.

Naegele's rule assumes a regular 28-day menstrual cycle with ovulation on day 14 — conditions that apply to only about 30% of women. Women with irregular cycles, PCOS (which can cause irregular ovulation), recent hormonal contraception use, or recent pregnancy or breastfeeding may have ovulation at significantly different times. For these individuals, LMP-based dating is less reliable, and first-trimester ultrasound dating is recommended to establish the EDD accurately.

This calculator cannot detect or account for variations in fetal growth rate, placental function, or medical conditions affecting pregnancy. The EDD produced here is a statistical estimate — normal birth occurs anywhere between 37 and 42 weeks. Some complications (intrauterine growth restriction, pre-eclampsia, placenta previa) may result in medically indicated preterm delivery before the EDD. Conversely, post-dates pregnancies (beyond 42 weeks) carry increasing risks and typically prompt clinical intervention. The calculator provides information for planning purposes only — all clinical decisions should be made with your OB/GYN or midwife.

This calculator is not a medical device and does not replace prenatal ultrasound for dating accuracy. If your care provider's ultrasound-derived EDD differs from the calculator's LMP-based EDD by more than 7 days in the first trimester, clinical practice generally adjusts the EDD to match the ultrasound measurement — the more accurate method. Follow your healthcare provider's established EDD for all clinical care decisions.