When should maternity leave begin based on a known last menstrual period (LMP) date?

Maternity Leave Timing Based on LMP

Maternity leave should typically begin at 36-38 weeks of gestation, which would be calculated as 252-266 days (36-38 weeks) from the first day of your last menstrual period (LMP). 1

Calculating Your Expected Delivery Date

• Add 280 days (40 weeks) to the first day of your LMP to determine your estimated date of delivery (EDD) using Naegele's rule, which is the standard method recommended by the American College of Obstetricians and Gynecologists 1, 2

• The actual median duration of pregnancy is 283 days from LMP (approximately 40 weeks and 3 days), with nulliparous women delivering slightly later at 284 days compared to multiparous women at 282 days 3

• Confirm your dating with first-trimester ultrasound (crown-rump length measurement before 14 weeks) if you haven't already, as this is more accurate than LMP dating alone and essential for proper timing 1

When to Start Maternity Leave

Standard Timing Framework

• Most women begin maternity leave between 36-38 weeks gestation to allow adequate rest before delivery while avoiding premature work cessation 1

• Calculate this as 252-266 days from your LMP (36-38 weeks × 7 days)

• Do not plan work commitments beyond 41 weeks gestation (287 days from LMP), as induction of labor is recommended at 41 weeks for post-term pregnancy to reduce maternal and fetal risks 4

Medical Considerations That May Require Earlier Leave

• If you develop gestational hypertension or preeclampsia, delivery is recommended at 37 weeks, so maternity leave should begin by 36 weeks at the latest 4

• For high-risk conditions (diabetes with poor control, fetal growth restriction, or other complications), coordinate leave timing with your obstetrician as delivery may be indicated at 37-39 weeks depending on the specific condition 4

Important Caveats

• Manual pregnancy wheels are notoriously inaccurate (up to 4 days error) - use electronic calculators or have your healthcare provider confirm the calculation 5

• If your menstrual cycle is not 28 days, the standard Naegele's rule requires adjustment - add or subtract the difference in days from 28 to get an accurate EDD 6

• Seek immediate prenatal care if you haven't established it yet, as comprehensive dating ultrasound and risk stratification are essential for accurate planning 1