What is the projected due date and what to expect during pregnancy, given the last menstrual period (LMP) was on a specific date?

Calculating Due Date and Pregnancy Expectations

To calculate the projected due date, add 280 days (40 weeks) to the first day of the last menstrual period (LMP), which provides the estimated delivery date based on standard obstetric practice. 1

Due Date Calculation Method

The gestational age is calculated from the first day of the last menstrual period, with pregnancy duration conventionally reported as 280 days or 40 completed weeks. 2

• Gestational age (GA) refers to completed weeks from the first day of the last menstrual period, which is the standard method used in obstetric practice 2
• For assisted reproductive techniques, gestational age is calculated by adding 14 days to the number of completed weeks since fertilization 2
• Once the estimated due date is determined from LMP or ultrasound, it should be documented clearly in the medical record and discussed with the patient 1

Important Caveats About LMP Dating

• LMP-based estimates may be unreliable in some women, particularly those with irregular cycles 3
• First trimester ultrasound is generally considered more accurate than LMP alone for pregnancy dating, with ultrasound estimates being more accurate in over 50% of cases 4
• Dating by LMP assumes accurate recollection of the date and conception on day 14 of the cycle, while ultrasound performed before 24 weeks estimates gestational age within 5-14 days 2
• Ovulation timing is variable, occurring between days 9-20 of a typical 28-day cycle, which can affect LMP-based calculations 2, 5

When to Modify the Due Date

• Subsequent changes to the estimated due date should be reserved for rare circumstances and documented clearly 1
• If first trimester ultrasound differs significantly from LMP dating, the ultrasound measurement should generally take precedence 4
• Crown-rump length measurement is more accurate than mean sac diameter once an embryo is measurable 2

What to Expect During Pregnancy: Timeline by Gestational Age

First Trimester (Weeks 1-13)

Week 5 (approximately 3 weeks after conception):

• Gestational sac typically becomes visible on transvaginal ultrasound at 5 weeks GA 2
• This appears as a round or oval fluid collection with a hyperechoic rim 2

Week 5.5:

• Yolk sac becomes visible at approximately 5.5 weeks GA, appearing as a thin-rimmed circular structure within the gestational sac 2
• Presence of a yolk sac provides incontrovertible evidence of pregnancy 2

Week 6:

• Embryo becomes visible on transvaginal ultrasound at approximately 6 weeks GA 2
• Cardiac activity (rhythmic pulsations) can be detected at 6 weeks GA 2
• Cardiac activity should be documented with M-mode or cine clip 2

Week 7:

• The amnion (thin membrane surrounding the embryo) becomes visible at approximately 7 weeks GA 2

Weeks 1-10:

• The developing conceptus is termed an "embryo" from conception through 10 weeks 6 days GA 2

Week 11 onward:

• Beginning at 11 weeks 0 days GA, the conceptus is referred to as a "fetus" 2

Second and Third Trimesters (Weeks 14-40)

Term pregnancy definitions:

• Term period corresponds to 37 weeks 0 days through 41 weeks 6 days GA 6
• Prolonged pregnancy begins at 41 weeks 0 days GA 6
• Post-term pregnancy begins at 42 weeks 0 days GA 6

Delivery timing:

• Live birth is defined as complete expulsion or extraction after 20 completed weeks of gestational age 2
• Stillbirth is defined as fetal death after 20 completed weeks of gestational age 2

Key Clinical Considerations

Pregnancy Confirmation

• A woman can be reasonably certain she is not pregnant if she is ≤7 days after the start of normal menses 2, 7
• Most qualitative urine pregnancy tests have a sensitivity of 20-25 mIU/mL 2, 5
• Serum pregnancy tests are more sensitive than urine tests and can detect hCG earlier 5
• Some studies show an additional 11 days past expected menses may be needed to detect 100% of pregnancies with qualitative tests 2, 7

Important Pitfalls to Avoid

• Do not use terms like "fetal pole" or "embryonic pole" as these are obsolete 2
• Avoid terms like "viable," "nonviable," or "viability scan" in the first trimester, as viability refers to ability to survive in extrauterine environment 2
• What may appear as a "false period" could be implantation bleeding, which is typically lighter and shorter than normal menstruation 7
• For women reporting bleeding with a positive pregnancy test, consider implantation bleeding rather than true menstrual period 7

Documentation Requirements

• The best obstetric estimate of gestational age should be recorded on the birth certificate and used for clinical care 1
• For research and surveillance purposes, the best obstetric estimate rather than LMP alone should be used 1