Since it is difficult to pinpoint the exact day at which a woman conceived, health professionals conventionally refer to the length of pregnancy as the time that has passed since the first day of the last menstrual period (LMP) which is the beginning of the menstrual cycle. The LMP is the correct way to express the number of weeks of pregnancy, the stage of fetal development and the age of fetal viability.
The length of pregnancy by LMP should not be confused with the gestational age, which is the time elapsed since conception. The gestational age is two weeks less than the conventional length of the pregnancy because conception takes place in the middle of the menstrual cycle, within 12 hours after ovulation.
FETAL DEVELOPMENT FROM THE LAST MENSTRUAL PERIOD (LMP)
(Reference: Management of Unintended and Abnormal pregnancy, Paul, Lichtenbeerg, Borgatta,Stubleld and Crenin. Wiley Blackwell Publishing Ltd, 2009):
Weeks 1 and 2 LMP: it might seem strange, but you are not actually pregnant the first two weeks of pregnancy. You conceive at the time of ovulation, which typically occurs in the middle of the menstrual cycle or about two weeks after the onset of your last menstrual period (LMP). Health professionals will count ahead 40 weeks from the start of LMP. This means your menstrual period and the time before you ovulate are counted as part of your pregnancy - even though you weren't pregnant at the time.
Week 3 LMP: fertilization occurs at the beginning of this week. The resulting one-celled entity is called a zygote which starts traveling down the fallopian tube towards the uterus.
Week 4 LMP: the rapidly dividing ball of cells - now known as the blastocyst - burrows into the uterine wall for nourishment. This is called implantation. The blastocyst separates into two sections. The inner part will become the embryo. The outer mass will become the placenta. At this stage, the embryo is invisible to the naked eye and to the ultrasound exam.
Week 5 LMP: You are only one week late with your period. The ultrasound images are frequently non-diagnostic of pregnancy. To diagnose pregnancy, visualization of no-embryonic-like parts or embryonic dependencies is required. During this week, organo-genesis of the brain, spinal cord and heart is taking place. The embryo is about 0.1 cm or the size of a grain of sand.
Week 6 LMP: The size of the embryo is about 0.4 cm, like a small grain of rice. Heart activity may be identifiable by ultrasound.
Week 7 LMP: the embryo might be a little bigger than the top of a pencil eraser.
Week 8 LMP: the embryo is about 1.4 - 1.6 cm (about 1/2 inch) long and it is conventionally called a fetus from 8 weeks until the time of delivery .
Week 11 LMP: the fetus measures about 2 inches (50 mm)
Week 12 LMP: the fetus is 5.3 cm long.
The morphology and anatomy of the embryo and fetus are important for you to know about, and so are fetal viability (the ability of the fetus to survive outside the uterus); and neurological disabilities in very pre-term newborns FETAL VIABILITY
is defined as the capability of a fetus that is sufficiently mature to live on its own and survive outside the uterus.
Because there are constant advances in the medical care of the newborn, the exact age of viability of a fetus is changing.
Fetal viability starts at a point between 24-30 wks LMP. The age of fetal viability is different for twins, small-for-gestational-age fetus, presence of congenital malformations, and/or lack of adequate medical care.
Abortion is defined as the expulsion of a non-viable fetus, before the fetus is mature enough to live on its own. Between 3-22 wks LMP the fetus is non-viable. First and Early Second Trimester Termination of Pregnancy thru 22 weeks LMP are currently performed when fetal viability is zero. The world's youngest surviving newborn was delivered at 21 weeks and 6 days by c/section in Florida, on October 24, 2007. This was the exception to the rule, as no newborn under 23 wks LMP had survived until then.
At 25 wks LMP the rate of survival of a newborn of is still under 30% These newborns are more likely to die (more than 70%) than to survive.
If we define viability as a 50% chance of survival outside the womb, currently the fetal viability is reached at about 26 weeks LMP.
The goal is fetal survival without severe neurological impairment.
Prior to 26 wks LMP, more than 70% of newborns are affected by serious neurological disabilities, the most devastating of which is cerebral palsy..
After 28 wks LMP, the survival rate is over 90%.
After 32 weeks LMP, the survival rate is 95% and the risk of neurological disability is substantially reduced.
The best possible outcome is achieved by delivery at/after 39 wks LMP, but not past 42 wks LMP .
NEUROLOGICAL DISABILITIES IN PRE-TERM NEWBORNS:
Over 70% of survivors born before 26 wks LMP are affected by serious neurological disabilities, the most devastating of which is cerebral palsy.
If you would like more information, you may contact us at any of the following numbers:
Toll Free: (800) 867-1693 Local calls: (954) 772-HELP (4357) or (954) 772-0933
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Our address is: 2100 E Commercial Blvd, Fort Lauderdale, Florida 33308-3822
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