Estimate due date, conception date, and pregnancy progress from your last period.
Estimated due date is for informational purposes only. Babies can arrive before or after the due date, and medical dating may be updated by ultrasound or clinical assessment.
Your estimated due date (EDD) is one of the most important dates in pregnancy, shaping your entire antenatal care schedule. Due date calculation is a blend of science and estimation — the 40-week benchmark is based on statistical averages, and only about 5% of babies are born on their actual estimated due date. This complete guide explains how due dates are calculated, the difference between LMP-based and ultrasound dating, what each trimester involves, and how NHS and US OB-GYN care schedules are structured around your EDD.
The most widely used due date calculation method is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in the early 19th century and still in use today:
EDD = First day of LMP + 280 days (40 weeks)
This can also be stated as: add 7 days and 9 months to the first day of your last menstrual period. For example, an LMP of 1 January gives an EDD of approximately 8 October. The formula assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is longer or shorter, the calculation should be adjusted — our due date calculator above does this automatically when you enter your cycle length.
Why 40 weeks from LMP but 38 weeks from conception? Because the 280-day count begins from the last period, not from conception. Conception typically occurs around day 14 of a 28-day cycle — two weeks after the LMP. So gestational age (as counted in obstetric practice) is always approximately 2 weeks ahead of fetal age from conception. When your midwife or obstetrician says "you are 10 weeks pregnant," they mean 10 weeks since your last period, not 10 weeks since the egg was fertilised.
LMP-based dating assumes a "textbook" 28-day cycle, but many women have cycles that are shorter, longer, or irregular. This is why ultrasound dating, performed at the first trimester dating scan, provides a more accurate EDD for many women.
During the dating scan (typically at 10–14 weeks), the sonographer measures the crown-rump length (CRL) of the embryo/fetus. Because fetal growth in early pregnancy is remarkably uniform across all pregnancies, the CRL measurement accurately predicts gestational age to within ±5 days before 14 weeks. If the ultrasound-calculated EDD differs from the LMP-based EDD by more than 5–7 days, the ultrasound date typically supersedes the LMP date in clinical practice.
In the UK, the NHS 12-week dating scan (offered between 10 and 14 weeks) establishes the official EDD that is used for all subsequent antenatal care planning. In the US, ACOG recommends that if the first ultrasound is performed before 14 weeks and dates differ from LMP by more than 7 days, the ultrasound date should be used.
Pregnancy is conventionally divided into three trimesters, each representing a distinct phase of fetal development and maternal experience:
| Trimester | Weeks (from LMP) | Key Developments |
|---|---|---|
| First trimester | Weeks 1–12 | Organ formation, embryo becomes fetus; miscarriage risk highest; nausea common |
| Second trimester | Weeks 13–26 | Rapid growth; baby movements felt (quickening, ~18–22 weeks); anomaly scan at ~20 weeks |
| Third trimester | Weeks 27–40+ | Weight gain, lung maturation, preparation for birth; frequent appointments |
Only about 5% of babies are born on their exact EDD. A "term" pregnancy spans weeks 37–42. Births are further categorised as:
First-time mothers (primiparous women) tend to give birth slightly later than average — approximately 40 weeks and 5 days — while women who have given birth before (multiparous women) tend to deliver closer to the EDD. Studies suggest that the normal range for full-term delivery spans roughly one week either side of the EDD.
The NHS in England offers a structured antenatal care programme outlined in NICE guideline NG201. For first-time mothers, approximately 10 appointments are offered; for women who have had previous pregnancies, approximately 7 appointments. Key appointments by gestational week include:
| Week | Appointment / Screening |
|---|---|
| 8–10 weeks | Booking appointment with midwife; blood tests, urine tests, blood pressure |
| 10–14 weeks | Dating scan (12-week scan); combined screening test for Down's syndrome if chosen |
| 18–20 weeks | Anomaly scan (20-week scan); checks fetal anatomy for structural issues |
| 25 weeks | Midwife appointment; bump measurement begins (symphysis-fundal height) |
| 28 weeks | Blood tests including full blood count and anti-D if Rh-negative; GTT if risk factors |
| 36 weeks | Baby's position checked; discussion of birth preferences |
| 41 weeks | Induction of labour offered if not yet delivered; CTG monitoring begins |
In the US, prenatal care is typically provided by an obstetrician (OB-GYN), midwife, or family physician. ACOG guidelines recommend the following schedule for low-risk pregnancies:
| Gestational Period | Appointment Frequency |
|---|---|
| Weeks 4–28 | Every 4 weeks |
| Weeks 28–36 | Every 2 weeks |
| Week 36 to delivery | Weekly |
Key US-specific screens include: cell-free DNA (cfDNA) testing or first trimester combined screen (10–13 weeks), anatomy ultrasound (18–20 weeks), Group B Strep culture (35–37 weeks), and glucose challenge test (24–28 weeks). Health insurance coverage under the ACA mandates coverage of prenatal services with no cost-sharing for most plans.
IVF pregnancies use modified due date formulas based on the embryo transfer date rather than LMP:
These calculations are used by UK fertility clinics and US reproductive endocrinologists. The advantage of IVF dating is precision — fertilisation date is known exactly, eliminating the uncertainty of estimated ovulation timing in natural cycles.
Twin and higher-order multiple pregnancies have different expected delivery timelines. Because multiple pregnancies carry higher risks of preterm birth, complications, and growth restriction, their "term" is considered earlier:
In the UK, the NHS offers enhanced surveillance for twin pregnancies, including more frequent scans starting at 16 weeks. In the US, ACOG recommends similar intensified monitoring for multiple gestations.
Beyond the clinical schedule, each trimester brings distinct physical and emotional changes. In the first trimester, nausea, fatigue, and breast tenderness are common as hCG levels rise sharply. The NHS advises all women to take 400 mcg of folic acid daily from pre-conception to 12 weeks to prevent neural tube defects; ACOG recommends 400–800 mcg in the US. The second trimester is generally more comfortable — nausea resolves for most women, energy returns, and the growing bump becomes visible. The anomaly scan at 18–20 weeks is a significant milestone. The third trimester brings back fatigue, back pain, and Braxton Hicks contractions as the body prepares for labour.
The standard formula (Naegele's Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation at day 14. If your cycle is longer or shorter, the estimate is adjusted accordingly. The NHS dating scan at 10–14 weeks uses ultrasound crown-rump length measurement to confirm or revise this date.
Gestational age counts from the first day of your last menstrual period (LMP) and is used in all clinical settings. Fetal age counts from the estimated date of conception, which is approximately 2 weeks after the LMP in a 28-day cycle. So a pregnancy described as "10 weeks gestational age" corresponds to approximately 8 weeks of fetal development.
The EDD is an estimate — only about 5% of babies are born on their exact due date. Most births occur within two weeks either side. Ultrasound dating at 10–14 weeks is accurate to within ±5 days. After 20 weeks, ultrasound dating is less accurate and the original EDD from the dating scan is not typically revised unless there is strong clinical reason.
The NHS dating scan (often called the "12-week scan") is offered between 10 and 14 weeks of pregnancy. It establishes gestational age, confirms the number of fetuses, and checks for some chromosomal conditions when combined with a blood test (combined first trimester screening). This scan sets the official EDD used throughout your pregnancy care.
In the UK, the NHS offers induction of labour at 41 weeks for pregnancies that have not yet delivered. After 42 weeks, the risk of complications for the baby increases. Induction may be offered earlier if there are clinical concerns. You will be offered increased monitoring (CTG cardiotocography) from 41 weeks if you choose to wait.
For IVF, the due date is calculated from the embryo transfer date: add 261 days for a Day 5 blastocyst transfer, or 263 days for a Day 3 transfer. This is more precise than LMP-based calculation because the exact fertilisation date is known. Your fertility clinic will provide your EDD at the point of transfer or at your first scan.
Twin pregnancies have an earlier target delivery date than singletons. Dichorionic-diamniotic (DCDA) twins are typically delivered at 37–38 weeks, and monochorionic-diamniotic (MCDA) twins at 36–37 weeks. These dates reflect the balance between fetal maturity and the increased risks associated with multiple pregnancy at later gestations.
A full term pregnancy is defined as 39–40 weeks + 6 days of gestation. "Term" as a whole spans 37–42 weeks. Babies born before 37 weeks are premature. The term "early term" (37–38+6 weeks) has been introduced to acknowledge that 37–38 weeks is technically term but not optimal — babies born at this stage have higher rates of respiratory and feeding difficulties than those born at 39–40 weeks.